A leaf that is sessile is attached directly to the plant without an intervening leaf stalk, also known as a petiole. Ferns are sessile as is trillium.

In the animal kingdom, sessility is the lack of motility. Sessile animals are unable to move on their own and are often attached to their surroundings. Coral and mussels are sessile.

The nearness of a sessile leaf to its stem and the rootedness of a sessile animal connotes squatness and solidity. Sessility is used in medicine to describe a tumor that is flat or has a broad base.

In A Deepness in the Sky Vernor Vinge references a sessile civilization, meaning a society that is closely bound to the land or a territory or its planet in contract to spacefaring civilizations. Vinge's book is where I first encountered the word.