Greek also had an alpha copulativum, a- or ha-, expressing union or likeness, which is the a- expressing "together" in acolyte, acoustic, Adelphi, etc. It is from PIE root *sem- (1) "one; as one, together with."
1660s, "existing or happening at the same time," from Late Latin synchronus "simultaneous," from Greek synkhronos "happening at the same time," from syn- "together" (see syn-) + khronos "time" (see chrono-). Meaning "recurring at the same successive instants of time" is attested from 1670s. Related: Synchronously.
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Definitions of asynchronous from WordNet
(digital communication) pertaining to a transmission technique that does not require a common clock between the communicating devices; timing signals are derived from special characters in the data stream itself;
not synchronous; not occurring or existing at the same time or having the same period or phase;