on·to 
(ŏnt′, -tə, ôn-)
(ŏnt′, -tə, ôn-)prep.
1. On top of; to a position on; upon: The dog jumped onto the chair.
2. Informal Fully aware of; informed about: The police are onto the robbers' plans.
adj.
Mathematics
Mathematics
Of, relating to, or being a function such that every element of the codomain is the value that corresponds to an element in the domain.