1 Literally, not synchronized; that is, proceeding at its own pace.
2 In electronics, it describes circuits that do not operate in step with other devices controlled by a shared CLOCK SIGNAL.
3 In communications, it refers to any communication PROTOCOL under which data transmission may start at any time - not just on a clock tick - and where the length of each data element is determined by markers (e.g. START BITS and STOP BITS) embedded in the data stream itself, rather than by any external timing constraint. An example is the common RS-232 serial protocol.
4 In a communications context, the term is sometimes used metaphorically to refer to any method of communication that does not require both parties to a discussion to be online at the same time (e.g. email, newsgroups, dial-up conferencing services).