A function \(f\) from set \(A\) to set \(B\) is called an *injection* and \(f\) is said to be *injective* if each element in \(A\) maps to a different element in \(B\).

In symbols, \(f\) is injective if \(f(a)=f(b)\) implies that \(a=b\).

An injective function is also known as a *one-to-one* or *one-one* (both read as ‘one to one’) function.