Kriya (in Sanskrit “action, deed, effort”) most commonly refers to a “completed action”, technique or practice within a yoga discipline meant to achieve a specific result. Types of kriya may vary widely between different schools of yoga. Another meaning of Kriya is the outward physical manifestations of awakened kundalini. Kriyas can also be the spontaneous movements resulting from the awakening of Kundalini energy.
Kriya Shakti is “a power of thought” said to be greatly studied by yogis. Among the specific Kriya practices are: Kriya Yoga and other techniques. Kriya Yoga was revived by the great master Mahavatar Babaji and the original lessons are available through sri yoganada swami’s kriya yoga institute (Self Realization Fellowship).
The Hatha Yoga Pradipika describes the six kriya cleansing techniques. They should only be performed if there is an excess of fat or phlegm in the body.
Definition courtesy Wikipedia.