$(L \dashv R)
\colon\;
X
\underoverset{R}{L}{\rightleftarrows}
Y$

there is a natural transformation (or more generally, a $2$-morphism) $\eta\colon id_X \to R \circ L$, called the unit of the adjunction. (A reason for the name is that $R \circ L$ is a monad, which is a kind of monoid object, and $\eta$ is the identity of this monoid. Since ‘identity’ in this context would suggest an identity natural transformation, we use the synonym ‘unit’.)

Similarly, there is $2$-morphism $\epsilon\colon L \circ R \to id_Y$, called the counit of the adjunction. (This is the co-identity of the comonad$L \circ R$.)

An adjunct is given by precomposition with a unit or postcomposition with a counit.

The left adjoint $L : X \to Y$ is fully faithful if and only if the unit $\eta : id_X \to R \circ L$ is a natural isomorphism.

Relation to monads

Every adjunction$(L \dashv R)$ gives rise to a monad$T \coloneqq R \circ L$. The unit of this monad$id \to T$ is the unit of the adjunction, $id \to R \circ L$.