What is the precise definition of “cadence” in astronomy?

I don't think there is an exact definition. The term is used as a means of referring to an array of observation times of some astronomical phenomenon.

In common usage, the terms "long cadence", or less often "low cadence", means that there is generally a longer time interval between observations. On the other hand, a "short cadence" or "high cadence" means a shorter time interval between observations (note the possible confusion here, but I can't be held responsible for common usage!).

Cadence is distinct from sampling rate or duty cycle, since the cadence can be irregular or non-periodic, either by design or circumstance. To me, if the observations are periodic, then it makes more sense to talk about a sampling interval or sampling frequency