In telecommunication, the term isochronous has the following meanings:

1. Of a periodic signal, pertaining to transmission in which the time interval separating any two corresponding transitions is equal to the unit interval or to a multiple of the unit interval.
2. Pertaining to data transmission in which corresponding significant instants of two or more sequential signals have a constant phase relationship.

Note: "Isochronous" and "anisochronous" are characteristics, while "synchronous" and "asynchronous" are relationships.

Source: from Federal Standard 1037C and from MIL-STD-188