The state of having no weight — the noun for the condition weightless describes. It is mostly a science and space word: astronauts train for weightlessness in a big water tank, and long periods of weightlessness make muscles and bones grow weaker. Because it is a noun you use it for the condition itself, not for a person or thing: "the effects of weightlessness on the human body". Outside science it can describe a feeling — the weightlessness you feel when a lift drops quickly, or the pleasant weightlessness of floating in the sea. It is a long, formal-sounding word; in speech people more often just say "zero gravity" or "being weightless".