Samasana is an asana or posture of Raja Yoga of Patanjali
Raja Yoga of Patanjali consists of yama, niyama, asana, pranayama, pratyahara, dharana, dhyana and Samadhi. The third, viz., asana, is posture/position of the body while practicing yoga. Two essential characteristics of posture are that it should be steady and comfortable. By asana one is freed from the tortures of duels such as pleasure and pain, profit and loss, etc. (Patanjali’s Yoga Sutra II.46-48). There are many postures and Yoga texts such as Gheranda Samhita, Hatha Yoga Pradipika, etc., and minor Upanishads explain them. Patanjali does not name them nor does he explain them.
One of the asana is samasana. Its importance is that it is a substitute or an alternative for two major postures, viz., padmasana and siddhasana.