mean time (Noun) — (astronomy) time based on the motion of the mean sun (an imaginary sun moving uniformly along the celestial equator).
mean time (Noun) — The time between one event, process, or period and another. ex. "meantime the socialists are running the government"
mean time (Adverb) — During the intervening time. ex. "meantime he was attentive to his other interests" ex. "in the meantime the police were notified"