What is Prāṇāyāma?
The two words broken down 'Prāṇā' and 'Ayāma' mean the following; 'Prāṇā' is the subtle life force. It also means breath, respiration, vitality and energy. 'Ayāma' is control. Or restraint. Extension. To stretch, regulate, expand or prolong.
The word 'Prāṇāyama' itself simply means the expansion of this subtle life force. It means prolongation of breath and its restraint. The śivasaṁhitā (one of the earliest texts on hatha yoga) calls it vāyu sādhana (vāyu = breath; sādhana = practice, quest).