meantime (n.)

also mean time, mid-14c., mene-time, "interim, interval between one specified time and another" (now only in in the mean time), from mean (adj.2) "middle, intermediate" + time (n.). Late 14c. as an adverb, "during the interval (between one specified time and another)." As a noun, properly written as two words but commonly as one, after the adverb. In the mean space "meanwhile" was in use 16c.-18c.

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Definitions of meantime from WordNet
meantime (n.)
the time between one event, process, or period and another;
Synonyms: interim / meanwhile / lag
meantime (adv.)
during the intervening time;
in the meantime the police were notified
meantime he was attentive to his other interests
Synonyms: meanwhile / in the meantime