Requires the use of tactics. Although not much different from strategy on brief glance, tactics is more involved with operation of individual units rather than squads/platoons, resource management or construction.
Alternate name: Tactics
The first video game about Tactical was released in 1982.
* Characters die easily
* Use of cover is mandatory (if there's shooting involved) even if there's no separate cover system
* Facing is important
* Characters/vehicles can't be restored in health or is slow, difficult, or limited in use
* No mid-combat/-mission recruiting, base building, etc.
Simplification: tactical games focus more on single unit movement within battle, such as which targets to fire at, where to take cover, etc., as opposed to strategy games which involve more about resource management and where the masses of troops need to be rather than what those masses of troops do there. Usually there's very little maneuvering in strategy games beyond larger strategies like flanking with secondary force, though similar can be done in smaller scale in unit tactics.
For those who are eager to add this to most games, consider this: can you win the game with superior numbers alone? or can you win with significantly inferior numbers? (or conversely, can you lose to a significantly smaller force? this thought experiment also involves taking into account the relative "power" of the forces). The latter case would say the game involves tactics, though it could also mean something unpleasant about your opponent. Tactical games are recognizable by the fact that economic supremacy does not dictate who wins, even if it does make it easier.
Non-strategy type games that are tactical usually feature heavy use of cover even if there's no specific cover mechanics in place, limited ability to see beyond corners without endangering yourself, lack of regenerating health and otherwise very fragile characters (fairly realistic damage model, 1-3 shots to kill depending on where the shot hits), enemies generally are not of poor skill (can actually aim), and so forth.