^I've always taken "regular" to mean someone who appears in, or at least is credited in the main cast list of, every episode. The supporting TOS players were never more than semi-regulars. Even Kelley was missing from two first-season episodes. Nichols was in 65 episodes, Doohan in 64, but Takei in only 51 episodes and Koenig in only 35.
We're obviously just going back and forth over semantics here, but I would point out that your definition doesn't fit certain contemporary series like The West Wing
which had large ensembles of regulars, but didn't feature every one of those actors in each episode.