Categories for the Working Mathematician says
Definition. A monad $T= \langle T, \eta, \mu\rangle $ in a category $X$ consists of a functor $T: X \to X$ and two natural transformations
$$\eta : I_X \Rightarrow T, \mu : T^2 \Rightarrow T $$
which make the following diagrams commute
We shall thus call $\eta$ the unit and $\mu$ the multiplication of the monad $T$; the first commutative diagram of (2) is then the associative law for the monad, while the second and third diagrams express the left and right unit laws, respectively.
Why is $\eta$ called the unity, and $\mu$ multiplication?
How is the first diagram the associative law? It says $μ∘Tμ=μ∘μT$, which equates $Tμ$ and $μT$ up to $μ$, so seems to me the commutative law instead.
Why are the second and third diagrams the second the left and right unit laws? They say that $μ∘ηT=μ∘Tη=1_T$, They seem to me that $ηT$ and $Tη$ are the same up to $\mu$ and they are both inverse of $μ$.
Thanks.
Given a category $\mathcal C$, the category $[\mathcal C,\mathcal C]$ of endofunctor is naturally a (non symmetric) strict monoidal category: the tensor product is the composition, the unit object if the identity functor and the associators and unitors are actual identities.
A monad on $\mathcal C$ is exactly a monoid in this monoidal category $[\mathcal C,\mathcal C]$.