"Upaya" is Sanskrit for "skillful means" or "method." It can refer to any activity, skill, experience or practice that helps someone toward the realization of enlightenment. For example, ritual can be a upaya.
Upaya is sometimes translated "expedient means." In some circumstances even lies and trickery can be upaya, if they help someone wake up to realization.
Upaya also refers to the ability of a bodhisattva to help others realize enlightenment.
In Buddhist literature, upaya often is paired with "prajna," meaning "wisdom," particularly the wisdom of shunyata. Upaya is the activity of prajna, which manifests as compassion (karuna). In Pure Land Buddhism, upaya, prajna and karuna are called the Three Gates to Liberation.