The movement of specific sub-units or vehicles on the battlefield. "Tactics" win battles, "Strategy" wins wars. (Or, according to the RLC, "Logistics" wins both - actually true).
To go Tactical (for example, on excercise) is all lights extinguished, no smoking, cold food, weapons ready, cam on. A Tactical Communications Vehicle, for example, will use an antenna no more complex than a sloping wire (normally lobbed over a handy branch) and be able to drive away as soon as possible.
More scary - a tactical nuclear weapon is low-yield, typically ten kiloton or less and is fired by (amongst others) MLRS; larger-bore tube artillery, aircraft etc. For example, tactical nuclear warheads were supposed to be used if the Soviets took certain positions in Europe. Additionally, if Keflavik AFB (Iceland) was taken, the book called for it to be wiped out with a nuclear warhead.