Close combat occurs in hand-to-hand combat when two combatants are well within arms-length of each other. Grappling, body-checking, stabbing and dirty tricks were all historically used in close combat.
Close Combat in the SCA
Since most (but not all) of the above actions are considered unsafe, and forbidden in the SCA's rules of the list, most fighters do not engage in this sort of extremely close combat, preferring instead to strike from beyond arms-length, which allows for better precision and mobility.
Close Combat in Re-Enactment
Similarly such variety of activities is also limited in re-enactment combat, unless agreed to beforehand. This type of activity typically occurs between individuals or small groups rather than a mass combat.