Kriya most commonly refers to a "completed action", technique or practice within a yoga

Yoga

Yoga is a physical, mental, and spiritual discipline, originating in ancient India. The goal of yoga, or of the person practicing yoga, is the attainment of a state of perfect spiritual insight and tranquility while meditating on Supersoul...

 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

Kundalini

Kundalini literally means coiled. In yoga, a "corporeal energy" - an unconscious, instinctive or libidinal force or Shakti, lies coiled at the base of the spine. It is envisioned either as a goddess or else as a sleeping serpent, hence a number of English renderings of the term such as 'serpent...

. Kriyas can also be the spontaneous movements resulting from the awakening of Kundalini

Kundalini

Kundalini literally means coiled. In yoga, a "corporeal energy" - an unconscious, instinctive or libidinal force or Shakti, lies coiled at the base of the spine. It is envisioned either as a goddess or else as a sleeping serpent, hence a number of English renderings of the term such as 'serpent...

 energy.

Kriya Shakti

Shakti

Shakti from Sanskrit shak - "to be able," meaning sacred force or empowerment, is the primordial cosmic energy and represents the dynamic forces that are thought to move through the entire universe in Hinduism. Shakti is the concept, or personification, of divine feminine creative power, sometimes...

is "a power of thought" said to be greatly studied by yogis. Among the specific Kriya practices are: Kriya Yoga

Kriya Yoga

Kriya Yoga finds mention in the ancient spiritual texts of Patanjali Yogasutras "Tapah svadhyayeshvara pranidhani kriyayogah" . It was later revived by Yogiraj Sri Shyamacharan Lahiri in the 19th century. Subsequently Paramhansa Yogananda in his Autobiography of a Yogi reported the same for his...

 and other techniques. Kriya Yoga was revived by Mahavatar Babaji and various schools, including Sri Yogananda Swami's kriya yoga institute (Self Realization Fellowship) claim to teach the original techniques.

The Hatha Yoga Pradipika

Hatha Yoga Pradipika

The Hatha Yoga Pradipika is a classic Sanskrit manual on hatha yoga, written by Svami Svatmarama, a disciple of Swami Gorakhnath. Said to be the oldest surviving text on the hatha yoga, it is one of the three classic texts of hatha yoga, the other two being the Gheranda Samhita and the Shiva...

 describes the six kriya cleansing techniques. They should only be performed if there is an excess of fat or phlegm in the body.

• neti
  Neti (Hatha Yoga)
  Neti is an important part of Shatkarma , the yogic system of body cleansing techniques. It is intended mainly to the cleaning of the air passageways in the head...
  
  
• nauli
  Nauli
  Nauli is one of the Kriyas or Shatkarma from Yoga. The exercise is to serve the cleaning of the abdominal region - digesting organs, small intestine and is based on a massage of the internal belly organs by a circular movement of the abdominal muscles.Nauli is an exercise of the classical Hatha...
  
  
• trataka
  Trataka
  Trataka is the practice of staring at some external object. It is used in yoga as a way of developing concentration, strengthening the eyes, and stimulating the Ajna chakra....
  
  
• kapalabhati
  Kapalabhati (Hatha Yoga)
  Kapalabhati , is an important part of Shatkarma , the yogic system of body cleansing techniques.The word kapalbhati is made up of two words, kapal meaning skull and bhati means shining, illuminating...
  
  
• dhauti
• basti
  Basti (Hatha Yoga)
  Basti is an important part of Shatkarma , the yogic system of body cleansing techniques. It is intended mainly to the cleaning of the lower abdomen, especially the colon. Both the Hatha Yoga Pradipika and other sources usually attribute to it many beneficial effects...