Source : Webster's Revised Unabridged Dictionary (1913)

Asynchronous \A*syn"chro*nous\, a. [Gr. ? not + synchronous.]
   Not simultaneous; not concurrent in time; -- opposed to
   {synchronous}.

Source : WordNet®

asynchronous
     adj 1: (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
            [ant: {synchronous}]
     2: not synchronous; not occurring or existing at the same time
        or having the same period or phase [ant: {synchronous}]

Source : Free On-Line Dictionary of Computing

asynchronous
     
         Not synchronised by a shared signal such as
        {clock} or {semaphore}, proceeding independently.
     
        Opposite: {synchronous}.
     
        1.  A {process} in a {multitasking} system
        whose execution can proceed independently, "in the
        {background}".  Other processes may be started before the
        asynchronous process has finished.
     
        2.  A communications system in which data
        transmission may start at any time and is indicated by a
        {start bit}, e.g. {EIA-232}.  A data {byte} (or other element
        defined by the {protocol}) ends with a {stop bit}.  A
        continuous marking condition (identical to stop bits but not
        quantized in time), is then maintained until data resumes.
     
        (1995-12-08)