synovial fluid
noun [ U ] medical specialized uk/saɪˌnəʊ.vi.əl ˈfluː.ɪd/ us/saɪˌnoʊ.vi.əl ˈfluː.ɪd/
（关节处的）滑液
a transparent, sticky liquid produced in our joints (= places where two bones are connected) that allows the bones and tendons to move smoothly