AC stands for alternating current, which is a type of electrical power where the flow of electricity rapidly switches direction many times per second. In the United States, AC electricity reverses direction 60 times each second.

This might sound strange: why would electricity flow backward and forward instead of just going one way? Thomas Edison believed electricity should flow in one direction, which he called direct current or DC. But another inventor, Nikola Tesla, showed that AC worked better for sending power over long distances. Today nearly all the electricity in your home is AC power.

When electricity flows from a power plant to your house through wires, it's AC. The outlets in your walls deliver AC power. Most large appliances, lights, and devices run on AC. However, many smaller electronics like phones and laptops actually need DC power, so they use adapters (those chunky plugs) to convert AC into DC.

You'll also see AC used as shorthand for air conditioning, which cools buildings. When someone says “turn up the AC,” they mean the air conditioner.