Simple Definition:

Celibacy is the practice of voluntarily abstaining from sexual activity (not participating in any sexual acts) and refraining from marriage or intimate relationships.

For example, a priest takes a vow [religious commitment] of celibacy and does not engage in sexual activities or marry. Similarly, a person who chooses to remain single and not have sexual relationships can be said to live a celibate lifestyle.

Very Simple Definition:

Celibacy is when someone decides not to have sex or get married.

For example, a monk who for religious reasons doesn’t have sex or get married can be considered celibate. Similarly, someone who doesn’t want to get married or have babies can also be described as living a celibate life.