Hyperconjugation is a concept in organic chemistry that explains why increasing the number of alkyl substituents on a carbocation or radical center leads to an increase in stability. It refers to the delocalization of electrons with the participation of bonds of primarily σ-character. The interaction of the electrons in a sigma (σ) orbital (e.g. C–H or C–C) with an adjacent unpopulated non-bonding p or antibonding σ* or π* orbitals gives a pair of extended molecular orbitals. Increased electron delocalization associated with hyperconjugation increases the stability of the system.

Hyperconjugation can be used to rationalize a variety of chemical phenomena, including the anomeric effect, the gauche effect, the rotational barrier of ethane, the beta-silicon effect, the vibrational frequency of exocyclic carbonyl groups, and the relative stability of substituted carbocations and substituted carbon-centered radicals, and the thermodynamic Zaitsevs rule for alkene stability.

In summary, hyperconjugation is a concept that explains the increased stability of carbocations and radicals due to the delocalization of electrons with the participation of bonds of primarily σ-character.