The phrase "yoga sutras" refers to a collection of ancient teachings about yoga. It consists of short and concise statements or aphorisms that explain the philosophy and practices of yoga. Full definition
In - depth study of Yoga Philosophy with respect to ancient texts of Yoga Sutras of Patanjali, Bhagavad Gita, and Yoga Upanishads. (rishikulyogshala.org)
Maharishi Patanjali describes asana in Yoga Sutras as «Sthiram Sukham Asanam», which means that asana is to stay in position or pose with ease. (yogicwayoflife.com)
Patanjali Yoga Sutra defines Yoga in verse 2 of the first chapter. (yogicwayoflife.com)