Combat is fighting between two groups of armed forces. When you engage in combat, typically this means you engage in fighting that involves weapons.
Combat comes from the Latin com- meaning "together with" and battere meaning "to fight." Combat can be used as a modifier; for example, a combat zone is an area where fighting is taking place. The word combat can also be used to refer to the action of fighting against something to prevent it. An after-school program for kids is working to combat the issue of street violence by creating a safe space where kids can go as an alternative.