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.
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1092