Definition
Anti-markovnikov is an addition reaction in which a hydrogen atom is installed at the more substituted vinylic position and another group (such as a halogen) is installed at the less substituted vinylic position.
Example
Anti-Markovnikov Addition
In one sentence
Anti-markovnikov is an addition reaction in which a hydrogen atom is installed at the more substituted vinylic position and another group (such as a halogen) is installed at the less substituted vinylic position.