A set of viewpoints and educational policies under which abstinence - abstaining from sexual activity - is the only form of sexual conduct that young people are taught. In theory, this teaches kids not to have sex, and prevents them using a more detailed curriculum as a checklist of things to do. In practice, kids who are taught abstinence only have a slightly lower rate of sexual activity, but aided by their enforced ignorance, have similar rates of pregnancy and sexually transmitted diseases as their more thoroughly educated peers.
In theory, abstinence only education should reduce the risk of sexual activity, but in practice, it does not. "In theory, there's no difference between theory and practice; but in practice, there's always a difference." - Y. Berra