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Asynchronous (from Greek, asyn 'not with', chronous 'time')
Not periodic.

Synchronous (from Greek, syn 'with', chronous 'time')
Periodic, signal in phase with clock, safe to sample signal with clock.

Mesochronous (from Greek, mesos 'mid/intermediate', chronous 'time)
Periodic, signal has the same frequency as the clock, but is potentially out of phase. It is safe to sample the signal if the clock signal is delayed by a constant amount.

Plesiochronous (from Greek, plesos 'close', chronous 'time')
Signal is nearly the same frequency and the clock, thus the phase between the clock and the signal varies slowly. It is safe to sample the signal of the clock is delayed by a variable amount. The difference in frequency can lead to dropped or duplicated data.

Heterochronous (from Greek, hetero 'different', chronous 'time')
Different clock and signal frequency, constant or varying phase.

Homochronous (from Greek, homo 'same', chronous 'time')
The relationship between two signals such that their corresponding significant instants are displaced by a constant interval of time.

Isochronous (from Greek, isos 'equal', chronous 'time')
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.
Pertaining to data transmission in which corresponding significant instants of two or more sequential signals have a constant phase relationship.

Anisochronous (from Greek, anisos 'unequal', chronous 'time')
Pertaining to data transmission in which the time interval separating any two significant instants in sequential signals is not necessarily related to the time interval separating any other two significant instants.

• The Designer's Guide to VHDL, 2nd Edition
• The Designer's Guide to VHDL
• The Student's guide to VHDL