two-way mirror
noun [ C ] uk/ˌtuː.weɪ ˈmɪr.ər/ us/ˌtuː.weɪ ˈmɪr.ɚ/
双向镜（从一面看是镜子，从另一面看是透明的）
a mirror that can be used in the usual way from one side but is transparent from the other side and can therefore be used to watch people without them knowing