The three-coordinate carbon atom in a
carbenium ion to which the excess positive charge of the ion (other than that located on heteroatoms)
may be formally considered to be largely attributed, i.e. which has one vacant p-orbital.
(N.B. It is not always possible to uniquely identify such an atom.) This formal attribution
of charge often does not express the real charge distribution.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1092