A person or object is weightless when they are in space and the earth's gravity does not affect them, so that they float around. Helen described life in a weightless environment during her period in space. something that is weightless seems to have no weight, especially when it is floating in space or water
having little or no weight or apparent gravitational pull; light; "floating freely in a weightless condition"; "a baby bat fluffy and weightless as a moth"; "jackets made of a weightless polyester fabric"