Isochronous transmission transmits asynchronous data over a synchronous data link so that individual characters are only separated by a whole number of bit-length intervals. This is in contrast to asynchronous transmission, in which the characters may be separated by arbitrary intervals, and with synchronous transmission [which does what?].
An isochronous message protocol assigns each data source a fixed amount of time to transmit (its "slot") within each cycle through the sources. That guarantees that each source will have regular opportunities to transmit the latest information. If a source has no more data to transmit, then the rest of its time slot is wasted. If it has more to send than will fit in its slot, it has to either store the excess data and transmit it in its next slot, or discard it.
Note that whether messages are isochronous or asynchronous is independent of whether the transmision of individual bits is synchronous or asynchronous.
[ANIXTER, LAN Magazine 7.93]