While the Sanskrit word yoga is commonly defined as «to join» or «to attach», many scholars have posited that in fact the paramount definition comes from Patanjali's Yoga Sutras (1:2, chitta vritti nirodhah): «The cessation of the fluctuations of the mind.» (sonima.com)
According to the Yoga Sutra, yoga is citta vritti nirodha; cessation of the fluctuations of the mind. (kimroberts.co)