Altitude is the height of something measured from a specific reference point, usually sea level or the ground. When a pilot reports their plane's altitude as 30,000 feet, they mean the aircraft is flying 30,000 feet above sea level. Mountain climbers track their altitude as they ascend, knowing that the air gets thinner and breathing becomes harder at higher altitudes.

You'll encounter it most often in aviation and geography. Denver, Colorado sits at an altitude of about 5,280 feet, earning it the nickname “The Mile High City.” Mount Everest reaches an altitude of 29,032 feet, making it Earth's highest point above sea level.

Altitude affects more than just how high something is. At higher altitudes, water boils at lower temperatures because air pressure decreases. Athletes training at high altitude develop more red blood cells to compensate for thinner air. Even baking recipes sometimes need adjustment at high altitude because cakes rise differently.

Don't confuse altitude with elevation (which usually refers to the height of land) or attitude (which describes someone's outlook or manner). When someone says a plane is “gaining altitude,” it means climbing higher into the sky.