What is perchlorite?

Answer 1

The ion [ClO4]-, assuming you meant perchlorate.

Polyatomic ions are named with differed prefixes and suffixes depending on the number of atoms bonded to their central atom.

Hypochlorite: [ClO]-

Chlorite: [ClO2]-

Chlorate: [ClO3]-

PerChlorate: [ClO4]-

("Per" is actually a shortening of "hyper", implying the chlorate is a "hyper" chlorate, with even more oxygen added to it.)