I consider it the average or the minimum/lull dps that you put out, depending on the terms its being compared to or the kind of fights its being consider for.

In (over-simplified) comparison, "burst" dps, for instance might end before your cooldowns have half cooled (always greater than "average") or until their durations end, while in "sustained" dps the cooldowns have half or more cooled (potentially average) since use, proportioning or devaluing their effect.

However, ramp-ups and other factors not allowing successive attacks, full use of debuffs, etc. would also come into consideration. While neither aspect is necessarily "burst" or "sustained", "sustained" would tend to lend more to the idea of "even if things don't go ideally".