An ELT Glossary : Adverb Particle

An adverb forming part of a phrasal or phrasal-prepositional verb - eg off, in and on in :

The plane took off slightly late.

He sent in the application .

I don't get on with John very well.

When the adverb particle is used in a transitive phrasal verb, it is separable : it may occur after the noun phrase and must do so if a pronoun is used :

He sent in the application.   Or  He sent the application in.

He sent it in.    But not   *He sent in it.