asynchronous describes activity that is not simultaneous or concurrent — participants act on their own schedules, and no exchange requires them to be present at the same moment.
In communication the word names a mode rather than a medium. Email, recorded video, comment threads, and written documents are asynchronous because the sender finishes their part and the receiver picks it up whenever they arrive. The cost is latency; the return is that no one has to negotiate a shared hour, and each participant can think for as long as the question deserves before answering.
In computing the term is older and more literal. Asynchronous transmission carries no shared clock — rather than timing characters against a common signal, the sending device marks the start of each one individually. The same idea generalizes to asynchronous execution, where a program issues an operation and keeps working instead of blocking until the result comes back.
The word is a negation of 📝synchronous, built with the Greek privative a- on syn-, "together," and chronos, "time." Not-at-the-same-time is the whole of the literal meaning, but in practice the word has come to carry the freedom that follows from loosening the timing, not merely the fact of it.