High impedance is expressed as “Hi-Z”. Specifically, it is a state that may occur during operation of a digital circuit in a MOS device when a signal from outside is cut off, or when an I/O alternate-function pin switches to input mode. This state is used to avoid signal collision when multiple outputs are connected to one signal line. Internally, only a very small leakage current flows along that signal line and “the signal line is in a floating state” is generally said.
Note that an input pin connected to a signal line which has no other driving sources becomes a floating state, and through-current may flow in the input buffer. In this case, the input pin requires special care for such as pull-up.