A day fighter is a fighter aircraft equipped only to fight during the day. More specifically, it refers to a multi-purpose aircraft that does not include equipment for fighting at night (such as a radar and specialized avionics), although it is sometimes used to refer to some interceptors as well.
Examples of planes that were classified as day fighters were the Supermarine Spitfire and Messerschmitt Bf 109. Both were excellent interceptors, but were also found in roles such as fighter-bomber and reconnaissance. However, neither was really able to be used as a useful night fighter, so the term day fighter was used to describe designs of this sort.
As the size and weight of night fighting equipment remained considerable in comparison to airframe size and engine power, the day fighter remained a part of front line air forces into the 1960s. In the immediate post-war era, designs like the F-86 Sabre, MiG-15 and Hawker Hunter typified the day fighter role. These were followed by similar supersonic designs in the early 1960s, including the F-104 Starfighter and MiG-21 Fishbed.
However, as the size of basic fighters grew, along with their engine power, the relative cost of carrying night fighting equipment fell until most designs carried such equipment. Lightweight fighters without full night equipment such as the F-5 Freedom Fighter/Tiger II are still used by nations which lack the money to acquire advanced fighter aircraft.
The last of the purpose-designed day fighters were the original F-16 and the YF-17 that competed for the Lightweight Fighter contract which had the goal of providing the USAF with an inexpensive day fighter that could be purchased in great numbers to establish air superiority. The designs originated in a mid-1960s effort known as the "Advanced Day Fighter". Ironically, after winning the contract the F-16 quickly matured into a much more capable multi-role aircraft and can no longer be considered a day fighter. Likewise, the YF-17 evolved into the much larger F/A-18, which is now the U.S. Navy's premiere all-weather combat aircraft.
See also 
- Britannica states "A day fighter is an airplane in which weight and space are saved by eliminating the special navigational equipment of the night fighter."
- "1942: Defence of Malta"
- "North American F-86 (Day-Fighter A, E and F Models)"