Yoga has many meanings including: union, systems of knowledge, logic.
In the Yoga Sutras of Patanjali (III/IV century BC) the fundamental book of Yoga, the eight limbs of yoga are described.

Yama (Rules of ethical and moral behaviour)
Nyama (Inner disciplines)
Asana (systematic practice of postures)
Pranayama (control of vital energy)
Pratyahara (control of the senses)
Dharana (concentration of the mind)
Dhyana (meditation)
Samadhi (self-realisation, supreme bliss)

The limbs are interconnected: practicing Ashtanga yoga means practicing all eight limbs to control the fluctuations of the mind (Yogas citta vritti nirodha. 1.1 Yoga Sutra) and achieve suprem consciousness (samadhi).