The definiton of an aberration is pretty abstract. So i'm crowdsourcing to you: 

* Makes an Aberration an Aberration? 

* What makes a Monstrosity a Monstrosity?

These seem pretty similar to me except I'm more likely to see an Aberration in Spelljammer or in Sigil and a Monstrosity on Prime M. plane in Toril or Greylance setting.  Is it mostly appearance and setting? 

Aberrations are often alien and otherworldly, often connected to the concept of the Far Realm, a place of chaos and madness. Monstrosities, on the other hand, are more commonly fantastical creatures with a basis in reality, but often magical or unusual in nature.

Aberrations are utterly alien beings, such as aboleths, beholders, flumphs, and mind flayers.

Monstrosities are unnatural creatures with strange origins, such as mimics and owlbears.

It's a little bit arbitrary, but as noted Aberrations typically originate from outside the Material Plane- usually they get associated with the Far Realm which is essentially just the cosmic "elsewhere" outside of the established cosmology, but there's a couple tied to other planes, like the Slaadi are Aberrations from the Plane of Limbo. Typically if it looks Lovecraftian and/or places a lot of emphasis on psychic stuff and isn't obviously something else like a psychic dragon, it's classed as an Aberration. Monstrosity is in many ways a catchall for anything that doesn't fit the vibe of the other types- it largely incorporates creatures created by magic on the Material Plane, cursed beings like Medusa and Lycanthropes, and even just obviously beefed up and fantastic but ostensibly "natural" wildlife like a Bulette.

Quote from Song_of_Blues >>

The definiton of an aberration is pretty abstract. So i'm crowdsourcing to you: 

* Makes an Aberration an Aberration? 

* What makes a Monstrosity a Monstrosity?

These seem pretty similar to me except I'm more likely to see an Aberration in Spelljammer or in Sigil and a Monstrosity on Prime M. plane in Toril or Greylance setting.  Is it mostly appearance and setting? 

I have always seen Aberrations as your Lovecratian varieties. 

Aberrations are typically natural creatures that are displaced or displaced themselves from their original habitat, to a new one that they are considered atypical in.

Monstrosities are typically unnatural, artificial and/or altered creatures in the habitat they exist in, regardless of origin.

Aberration & Monstrosity PLAYER Species, however, are either descended from or tied to existing aberrations and/or Monstrosities.

The upcoming retool Kalashtar are, for example, inexorably bound to the Quori, which are natural for the plane they live in but unnatural in Eberron's material plane & thus Kalashtar are influenced on a fundamental level into an Aberrant state, but for Thri-kreen as playable monstrosities, it's a lot more complicated.

Basically, Monstrosities are weird creatures that belong on the material plane, or were created by magic. Abberations came from somewhere else.

Ok, D&D can be weird about it, but there is a logic.
You can imagine it like this. 

If you create or encounter a creature that made through magic or genetic manipulation using things from the material plane, it is a Monstrosity. So like a mutant. 

If you create or encounter a creature that made through magic or genetic manipulation using things from outside the material plane, it is an Aberration. So like an ALIEN mutant. 

How is a naturally occurring monstrosity different from a beast? Well, that is also tricky but knowable. Magic as a force in the realms can cause mutations in otherwise normal creatures, turning them into monstrosites.  So Magic mutants, but not magic mutants from another world. 

Does it make sense? Only kinda, and i am sure it makes all the biologists in Faerun cry, and give them something to talk about.

In the definition for a monstrosity, the only part that matters is at the end: "a catch-all category for creatures that don't fit into any other type."

So, for example, non-undead creatures of the Shadowfell such as the nagpa, the shadow mastiff, the skulk or the sorrowsworn could have been grouped under a creature type called "umbral" or the like. But since 5e doesn't have that type, it just files them as monstrosities.

I think there is some lore issue here that I'm not understanding then. Like, i understand that Lovecraftian horrors are the classical aberrations, since they are explicity called out as "otherworldly" or "ancient beyond the gods" but there are so many monsters that seem to be in grey areas. 

So a Beholder and beholder-kin are not stated as being from other planes but they are "aberations", not "monstrosities."

A Yochlol is classified as "demon" even though they are very clearly the creation of a deity residing on another plane and don't get mentioned as being integral in any way to the Blood War with the devils. 

A Chuul, which is typically created in the Prime Material plane is an "aberration" even though its original stock "material" came from the Prime Mat plane. , i guess because it mutated into one by another aberration?  But a Drider, mutated from a humanoid by a deity residing in another plane, is somehow neither "demon" nor "aberration" but a "monstrosity."

An Otyugh is not associated with any other planes, is an "aberration." but a Remorhaz is more "normal" i guess, so is a "monstrosity" instead? And so is a Tarrasque, despite the Tarrasque having magic resistance and being so huge (and solitary) that it probably could only have been created by a deity.

A Merrow is a Prime Material fish-person transformed by Demogorgon, and is "monstrosity", not a "fiend" nor an "aberration."

So i guess the idea is that anything transformed or created by an aberration becomes an aberration even if it's material came from the Prime Material plane. But anything transformed by a deity or devil or a fiend is a monstrosity...except clearly in FRealms lore, devils and demons do create devils and demons, respectively, from the souls of creatures that had lived on the Prime Mat plane.

Quote from spamandtuna >>

Aberrations arise as a natural part of an unnatural environment. Mind Flayers are a normal part of the Far Realm, but for inhabitants of the Material Plane, the Far Realm itself is not normal.

Monstrosities arise as an unnatural part of a natural environment. Owlbears originated in the Material Plane, but they were created by mages, not by nature.

 So...Humans would be the Aberrations in the Far Realm?  I kind of like that idea, Aboleths fleeing in fear from some wayward teenager!

Quote from Song_of_Blues >>

I think there is some lore issue here that I'm not understanding then. Like, i understand that Lovecraftian horrors are the classical aberrations, since they are explicity called out as "otherworldly" or "ancient beyond the gods" but there are so many monsters that seem to be in grey areas. 

So a Beholder and beholder-kin are not stated as being from other planes but they are "aberations", not "monstrosities."

A Yochlol is classified as "demon" even though they are very clearly the creation of a deity residing on another plane and don't get mentioned as being integral in any way to the Blood War with the devils. 

A Chuul, which is typically created in the Prime Material plane is an "aberration" even though its original stock "material" came from the Prime Mat plane. , i guess because it mutated into one by another aberration?  But a Drider, mutated from a humanoid by a deity residing in another plane, is somehow neither "demon" nor "aberration" but a "monstrosity."

An Otyugh is not associated with any other planes, is an "aberration." but a Remorhaz is more "normal" i guess, so is a "monstrosity" instead? And so is a Tarrasque, despite the Tarrasque having magic resistance and being so huge (and solitary) that it probably could only have been created by a deity.

A Merrow is a Prime Material fish-person transformed by Demogorgon, and is "monstrosity", not a "fiend" nor an "aberration."

So i guess the idea is that anything transformed or created by an aberration becomes an aberration even if it's material came from the Prime Material plane. But anything transformed by a deity or devil or a fiend is a monstrosity...except clearly in FRealms lore, devils and demons do create devils and demons, respectively, from the souls of creatures that had lived on the Prime Mat plane.

 Some things are legacy beyond 5th edition.  Otyughs did have an elemental plane connection, as did Merrow iirc.

Yochol are definitely demons, "Handmaidens of Lloth" in most accounts where Lloth is a demon lord.  

Quote from Song_of_Blues >>

So a Beholder and beholder-kin are not stated as being from other planes but they are "aberations", not "monstrosities."

According to the DMG's description of the Far Realm, "Aberrations such as mind flayers and beholders are either from this plane or shaped by its strange influence". So beholders do have a particular planar origin, as do illithids. I think most other aberrations can also be assumed to have a Far Realm connection somewhere in their lore, if not in 5e then in some previous edition, the clear exception being the slaadi. I have no idea how the CN exemplars ended up being classified as aberrations.

With monstrosities, I wouldn't worry what plane they are from. Sorrowsworn embody the Shadowfell. Astral dreadnoughts are bound to the Astral Plane. Carrion crawlers aren't associated with any plane in particular. I maintain that monstrosity is just the "none of the above" option at the end of the list.

In 3e, "An aberration has a bizarre anatomy, strange abilities, an alien mindset, or any combination of the three".

In 4e, "Aberrant creatures are native to or corrupted by the Far Realm."

In 5e, "Aberrations are utterly alien beings. Many of them have innate magical abilities drawn from the creature’s alien mind rather than the mystical forces of the world."

For the most part, they are the same in all three editions, though 5e, oddly, reclassified Slaad (native to Limbo, no known connection to the Far Realm) as aberrations.

Regarding Slaadi, what else would they be? They're not humanoid, celestial, construct, fey, fiend, dragon, giant, ooze, plant, or undead. Between monstrosity and aberration, the latter has a stronger "denizens of another plane" vibe.