A carbocation ( /ˌkɑrbɵˈkætaɪ.ɒn/) is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability (octet rule). Therefore carbocations are often reactive, seeking to fill the octet of valence electrons as well as regain a neutral charge. One could reasonably assume a carbocation to have sp³ hybridization with an empty sp³ orbital giving positive charge. However, the reactivity of a carbocation more closely resembles sp² hybridization with a trigonal planar molecular geometry. An example is the methyl cation, CH₃⁺.

A carbocation was previously often called a carbonium ion but questions arose on the exact meaning. In present day chemistry a carbocation is any positively charged carbon atom. Two special types have been suggested: carbenium ions (protonated carbenes, hence the name) are trivalent and carbonium ions (protonated alkanes) are pentavalent or hexavalent. University level textbooks only discuss carbocations as if they are carbenium ions, or discuss carbocations with a fleeting reference to the older phrase of carbonium ion or carbenium and carbonium ions. One textbook to this day clings on to the older name of carbonium ion for carbenium ion and reserves the phrase hypervalent carbenium ion for CH₅⁺.