adjective (a)

• (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(adj.all)

  Antonym:
  synchronous
  

  source: wordnet30
• not synchronous; not occurring or existing at the same time or having the same period or phase(adj.all)

  Antonym:
  synchronal, synchronic, synchronous
  

  source: wordnet30
• Not simultaneous; not concurrent in time; -- opposed to synchronous.(adjective)

  source: webster1913