Microwave can mean two different things:

1. A microwave oven is a kitchen appliance that heats food using invisible waves of energy called microwaves. Unlike a regular oven that heats the air around your food, a microwave oven makes the water molecules inside your food vibrate rapidly, which creates heat from the inside out. That's why a microwaved potato can be hot in the middle while the plate stays cool. You can microwave leftovers for dinner or microwave popcorn for movie night. Microwave ovens became popular in the 1970s because they heat food much faster than conventional ovens.
2. In science, a microwave is a type of electromagnetic wave, similar to radio waves but with a shorter wavelength. These waves can be about as long as your finger (between one millimeter and one meter). Besides heating food, microwaves carry cell phone signals and WiFi data. Radar systems use microwaves to detect airplanes and weather patterns.