A PAR light bulb is a light with a parabolic aluminized reflector. The name PAR refers to the shape and design of the light bulb.
PAR light bulbs come in different sizes for lamps. It is measured by its diameter, or the length across the surface of the bulb. When naming a PAR light bulb size, manufacturers use the acronym "PAR," followed by a number that describes the diameter size. The number corresponds to eighths of a inch. Common sizes are PAR16, PAR20 and PAR30. PAR16 measures 16/8 of an inch in diameter, PAR20 measures 20/8, and PAR30 measures 30/8.