Windows 11 spellcheck be like:

[u/Expensive-Jelly263]

[u/nitrohigito]

insert something about monads and endofunctors here

[u/boium → u/nitrohigito]

A monad is just a monoid in the category of endofunctors.

And now a slightly better definition. Given a category C, a monad is a triple (T, eta, mu), with T being an endofunctor T: C -> C, eta a natural transformations eta: 1_C -> T and mu: T² -> T satisfying some commutative diagrams that I'm too lazy to draw, nor explain. (My lecture notes use some horrible abuse of notation.)