This is one of those topics that comes up once or twice a year. Tactical is really a subjective term used (mostly abused) for marketing gains. It's a term that really belongs to the military or police. That of should not be misconstrued to mean a civilian can't have any tactical things, of course they can. But when your life's on the line, you need your equipment to work as designed, and you need logically designed equipment.
So that would mean reliable and very powerful. That would also mean forward press silent momentary at FULL power. Note all of that - forward, silent, momentary and full power. Many lights with a twist function to lock them on are called tactical but under stress, while holding a firearm, and possibly even wearing gloves, a twist could be a hinderance, depending on your specific situation. Low isnt' very tactical if it's white - it would still give away your position. A tactical light doesn't "need" a low at all, especially if there is ANY chance you could end up in it accidentally. Anything that could hinder the light coming on full when you need it, UNDER STRESS, takes away from it being "tactical."
However, no one need agree with any of that because there are as many definitions as there are ******** (use your imagination!:devil:).