A carbocation is an ion with a positively-charged carbon atom. The carbon atom has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability. Therefore the carbon cation is unstable and very reactive, seeking to fill its octet of valence electrons as well as regain its neutral charge.

In organic chemistry, the carbocation is often the target of nucleophilic attack by nucleophiles like OH^- ions or halogen ions.

Carbocations are classified as primary, secondary, or tertiary depending on the number of carbon atoms bonded to the ionized carbon. Primary carbocations have one or zero carbons attached to the ionized carbon, secondary carbocations have two carbons attached to the ionized carbon, and tertiary carbocations have three carbons attached to the ionized carbon.

Primary carbocations are highly unstable and reactions such as the S_N1 reaction and the E1 elimination reaction normally do not occur if a primary carbocation would be formed. An exception to this occurs when there is a carbon-carbon double bond next to the ionized carbon. Such cations as allyl cation CH₂=CH-CH₂+ and benzyl cation C₆H₅-CH₂+ are quite stable and molecules which can form these carbocations are especially reactive. Secondary carbocations are formed more readily than primary carbocations, and tertiary carbocations are formed still more readily.