An injective function, or an injection is a function in math that does not map more than one element from its domain to its codomain. An injective function is often called one-to-one because there is only one way to reach any one element in the range.
An injection does not always reach every element in its codomain. The special ones that do are called bijections. For example, mapping houses to their pets is an injective function since a pet cannot belong to more than one house, but not every animal belongs to a house at all.