Sessility comes from the word sessile. In biology, it means to stay in one place on the ground. It is the opposite of motile. It is mostly used for marine and freshwater species. They often have larvae which are motile, float or swim, and adult forms which are stationary. Many sessile forms can move from place if they need to. These life forms choose a suitable place, and stay fixed as long as they can. They usually secrete some substance which holds them on.