Mean time and Time unit

Mean time noun - Astronomy) time based on the motion of the mean sun (an imaginary sun moving uniformly along the celestial equator.

Time unit and mean time are semantically related. In some cases you can use "Time unit" instead a noun phrase "Mean time".

Time unit noun - A unit for measuring time periods.

Mean time and time unit are semantically related. Sometimes you can replace phrase "Mean time" with "Time unit", this phrases are similar.