释义
n.
the central or most important source of motion or action
行动主因，原动力。
(in the medieval version of the Ptolemaic system)an outer sphere supposed to move round the earth in 24 hours carrying the inner spheres with it
第十层天（中世纪托勒密天动说中被认为二十四小时绕地球旋转、托载内层天体的外层天体）。
词源
" from medieval Latin, literally ‘first moving thing’. "