Where can I find the official defination of "creature" in the rules? Tried searching D&D Beyond ot no avail, glanced through the players hand book and the basic rules but nothing jumped out, googling brings up a lot of fan sites but nothing official.

Playing with new players and the DM keeps being asked if each new monster is a "creature"; to my understanding "creature" is just a catch all term for any living thing to differentiate from "object" but that's just my understnading, I want to know where the actual defination is written down so I can read it and show it to them.

This is just one of those things they never bothered to write down an in-game definition of due to their idea of using natural language because they assume that everyone knows what a "creature" is already.

Quote from 2000AD >>

to my understanding "creature" is just a catch all term for any living thing to differentiate from "object" but that's just my understnading,

Yep, anything that's up and about and moving around. Any humanoid, beast, demon, undead, construct or whatever, even a mimic qualifies.

Creature is any monster or NPC usually coming with a type. Objects in comparison don't. 

If you're inquiring about a specific thing you're not sure about fell free to post it here and you will get an answer quickly.

The Monster Manual defines a monster as "any creature that can be interacted with and potentially fought and killed."

As others have wrote.  There really is no creature.  it is just a generic universal term to lump together things that appear in the books.

Creatures would be Beasts,  Undead,  Fey, etc...

I'd give creature as anything with grammatical animacy.