ADC

League of Legends (LoL) players use ADC to mean "attack damage carry." ADC champs can deal a lot of damage with their basic attacks, and are responsible for carrying their team to victory.

ADCs often start out weaker than other champions but grow stronger over time (by building items and/or racking up kills). Eventually, their basic attacks deal hundreds of points of damage per second. Some examples of ADC champs include Ashe, Tristana, and Twitch.

In LoL, the term ADC has largely been replaced by the terms Marksman, Slayer, and Fighter. Marksmen are ranged ADC champs, while Slayers and Fighters are melee ADC champs. However, because players used ADC so commonly for so long, you'll likely still see League and other MOBA players refer to basic damage-dealing champs as ADCs.