The act of assuming or maintaining a seated position WordNet 3.0 & Open English WordNet

Sitting is a basic action and resting position in which the body weight is supported primarily by the bony ischial tuberosities with the buttocks in contact with the ground or a horizontal surface such as a chair seat, instead of by the lower limbs as in standing, squatting or kneeling. Wikipedia

Human resting position; body weight is supported primarily by the buttocks in contact with the ground or a horizontal object such as a chair Wikidata

A period during which one is seated for a specific purpose. Wiktionary