This is a technical term used in the Yogasūtras of Patañjali (200 B. C.) (vide 2.1).
Yoga is conquering the modifications of mind, ultimately resulting in samādhi (perfect concentration, superconscious experience).
Patañjali recommends kriyāyoga for the benefit of ordinary sādhakas, who are incapable of practising all the eight steps.
The three disciplines—tapas or auste-rity, svādhyāya or study of the scriptures as also the japa of praṇava (Om) and īśvarapraṇidhāna (surrendering the fruits of actions to God)—together form kriyā-yoga. They are called ‘kriyāyoga’ since they are in the form of kriyā (activity) and lead to yoga (union with God) in course of time.
Tapas or austerity consists of physical austerity like serving the elders and continence, austerity of speech like speaking the truth and the study of the Vedas, and, austerity of mind like purity of emotions.
Svādhyāya includes the study of scriptures that help in the path towards kaivalya (liberation) and the repetition of mantras like Om and the Gāyatrī.
Īśvarapraṇidhāna comprises offering all actions and their fruits at the feet of God.
This kriyāyoga helps in the attenuation of evil qualities and ultimately leads to samādhi.