Plain English Meaning
A cockpit gauge that figures out how high you are by measuring how much air pressure is pushing on it. Higher up means less pressure, so the gauge converts that pressure into a height reading you can read in feet.
Full Definition
An aneroid instrument that measures the height of the aircraft above a selected pressure reference by sensing the surrounding atmospheric pressure and converting it into an altitude reading in feet. The pilot sets the reference pressure in the altimeter's Kollsman window so the instrument displays altitude relative to that reference (typically the local altimeter setting, which produces height above mean sea level).