1. to a place or position on; upon; on: to get onto a horse.
  2. Informal. in or into a state of awareness about: I’m onto your scheme.


  1. Also surjective. Mathematics. pertaining to a function or map from one set to another set, the range of which is the entire second set.

  1. a combining form meaning “being,” used in the formation of compound words: ontogeny.


  1. to a position that is onstep onto the train as it passes
  2. having become aware of (something illicit or secret)the police are onto us
  3. into contact withget onto the factory

combining form

  1. existence or beingontogeny; ontology

prep.1580s, as on to, from on + to. Appeared much later than parallel into. As a closed compound (on analogy of into), first recorded 1715. word-forming element meaning “a being, individual; being, existence,” from Greek onto-, from stem of on (genitive ontos) “being,” neuter present participle of einai “to be” (see essence). pref.

  1. Organism; being:ontogeny.

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