A function$f$ from $A$ to $B$ is injective if $x = y$ whenever $f(x) = f(y)$. An injective function is also called one-to-one or an injection; it is the same as a monomorphism in the category of sets.

In constructive mathematics, a strongly extensional function between sets equipped with tight apartness relations is called strongly injective if $f(x) \ne f(y)$ whenever $x \ne y$ (which implies that the function is injective). This is the same as a regular monomorphism in the category of such sets and strongly extensional functions (while any merely injective function, if strongly extensional, is still a monomorphism). Some authors use ‘one-to-one’ for an injective function as defined above and reserve ‘injective’ for the stronger notion.