carbonium ion

https://doi.org/10.1351/goldbook.C00839
The term should be used with great care since several incompatible meanings are currently in use. It is not acceptable as the root for systematic nomenclature for @C00817@.
  1. In most of the existing literature the term is used in its traditional sense for what is here defined as @C00812@.
  2. A @C00817@, real or hypothetical, that contains at least one five-coordinate carbon atom.
  3. A @C00817@, real or hypothetical, whose structure cannot adequately be described by two-electron two-centre bonds only. (The structure may involve carbon atoms with a @C01331@ greater than five.)
Sources:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1093 [Terms] [Paper]
PAC, 1995, 67, 1307. (Glossary of class names of organic compounds and reactivity intermediates based on structure (IUPAC Recommendations 1995)) on page 1325 [Terms] [Paper]