According to the IUPAC, a "carbocation" is any cation containing an even number of electrons in which a significant portion of the positive charge resides on a carbon atom. Prior to the observation of five-coordinate carbocations by Olah and coworkers, "carbocation" and "carbonium ion" were used interchangeably. Olah proposed a redefinition of "carbonium ion" as a carbocation featuring any type of three-center two-electron bonding, while a "carbenium ion" was newly coined to refer to a carbocation containing only two-center two-electron bonds with a three-coordinate positive carbon. Subsequently, others have used the term "carbonium ion" more narrowly to refer to species that are derived (at least formally) from electrophilic attack of H+ or R+ on an alkane, in analogy to other main group onium species, while a carbocation that contains any type of three-centered bonding is referred to as a "non-classical carbocation". In this usage, 2-norbornyl cation is not a carbonium ion, because it is formally derived from protonation of an alkene (norbornene) rather than an alkane, although it is a non-classical carbocation due to its bridged structure. The IUPAC acknowledges the three divergent definitions of carbonium ion and urges care in the usage of this term. For the remainder of this article, the term "carbonium ion" will be used in this latter restricted sense, while "non-classical carbocation" will be used to refer to any carbocation with C–C and/or C–H σ-bonds delocalized by bridging.