Simply defined, a gish is a character that uses both swords and spells. I think originally they were specific to githyanki but the term is used to describe any fighter/mage character.

Quote from The_Ace_of_Rogues >>

Typically when you see people talking about gish builds here, it’s specifically a full caster who can be effective as a weapon user as well. As you might imagine, there’s a lot of criticism over the “effectiveness” of the weapon use, since as the number and level of spell slots grows the less desirable making a weapon attack looks.

 yuuuup. Right now I'm playing a Bladesinger in a seafaring campaign, but with the character flavor that the person she was inspired by was also a bladesinger, but she only ever got to see the melee side of things, so when I take spells as I level, I try to think more tactically vs. damage dealing; spells that give me more mobility on the battlefield, stuff that keeps us safe on the water, etc., etc.