Illustrated Glossary of Organic Chemistry

Carbocation: A contraction of carbon cation. A structure with a positive formal charge on carbon due to an open octet. Formerly called carbonium ion, but this term is considered outdated.

Carbocations can be categorized by the number of carbons directly bonded to the open octet carbon:

Methyl carbocation
No +C-C bonds

Ethyl carbocation
A primary (1o) carbocation
One +C-C bond

Isopropyl carbocation
Secondary (2o) carbocation
Two +C-C bonds

Tert-butyl carbocation
A tertiary (3o) carbocation
Three +C-C bonds