The A6M is universally known as the Zero from its Japanese Navy type designation, Type 0 Carrier Fighter, taken from the last digit of the Imperial year 2600 (1940), when it entered service. In Japan it was unofficially referred to as both Rei-sen and Zero-sen; Japanese pilots most commonly called their plane Zero-sen.
The meaning of the A6M official designation was that "A" signified a carrier-based fighter, "6" for the sixth such model built for the Imperial Navy, and "M" for the manufacturer, Mitsubishi.
The official Allied code name was "Zeke", in keeping with the practice of giving male names to Japanese fighters, female names to bombers, bird names to gliders, and tree names to trainers. "Zeke" was part of the first batch of "hillbilly" code names assigned by Captain Frank T. McCoy of Tennessee, who wanted quick, distinctive, easy to remember names. When in 1942 the Allied code for Japanese aircraft was introduced, he logically chose "Zeke" for the "Zero." Later, two variants of the fighter, not immediately identified as such, received their own code names: the A6M2-N (floatplane version of the Zero) was called Rufe and the A6M3-32 variant was initially called Hap. After objections from General "Hap" Arnold, commander of the USAAF, the name was changed to Hamp.