Sukha

Ease, comfort, well-being. The word Patañjali uses alongside sthira (steadiness) to define āsana: «sthira-sukham āsanam» — the posture is steadiness and ease.

Etymologically, su (good) + kha (space, the axle-hole of a wheel). A wheel that turns well has a good axle; a body that moves well has sukha. Its opposite is duḥkha (suffering) — literally a defective axle.

In practice, sukha is not the absence of effort but its quality. A posture has sukha when the body finds its place without unnecessary resistance, when breath flows without obstruction, when the mind can remain without struggle.

It is also one of the three qualities of an object in relation to the subject: that which generates pleasure or well-being.