the state of having or appearing to have no weight; the fact of not being affected by gravity (= the force that makes things fall to the ground)

The animals will be used to study the effects of weightlessness on the brain and nervous system. 这些动物将被用来研究失重对大脑和神经系统的影响。

After nearly five months of weightlessness, the first tenants of the international space station said they are eager to return to Earth. 在经历了近五个月的失重状态后，国际空间站的第一批住户表示，他们渴望返回地球。