1. [adj]  not synchronous; not occurring or existing at the same time or having the same period or phase
2. [adj]  (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 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.