AC

In Dungeons & Dragons (DnD) and other RPGs, AC stands for "armor class." A player character's (PC) armor class determines how hard they are to hit in combat, and a Dungeon Master (DM) will often ask players what their AC is while attacking with non-player characters (NPCs). NPCs also have ACs, and players' attack rolls must either meet or beat an NPC's armor class to hit.

In the 5th Edition of D&D, each player character's base AC is 10. This base AC can be modified in a number of ways, including by armor, spells, and class-based abilities. Thankfully, online character sheets (such as those available through DnDBeyond.com) can now track players' armor classes for them, allowing them to focus on casting spells rather than calculating sums.

If you live anywhere hot, AC is the gift that keeps giving. AC is the air cooling system in households, commercial buildings, arenas, vehicles, etc., that cools the space to keep it at a pleasant temperature.

Engineer Willis Haviland Carrier invented the first modern air conditioner in 1902. Roughly, an AC works by absorbing heat from the indoor air into the refrigerant, cooling the air as it passes over the coil, and the indoor unit pumps the chilled air back via a blower fan to the living space, vehicle interior, etc.

AC makes scorching hot climates (dry and humid) livable. For example, if you are in Houston on a hot, muggy day, you are probably having trouble breathing and searching for the nearest place with AC before you pass out.