40 (forty) is a positive integer following 39 and preceding 41. Its ordinal form is written "fortieth" or 40th.
- 40 is the 27th composite number.
- 40 is an even composite number.
- 40 is the only number whose letters are in alphabetical order.
- 40 is a number with four prime factors.
- In Greek-based number-naming systems, 40 is associated with the prefix "tetraconta-", and with the prefix "quadraginti-" in Latin systems.
- Nirvana Supermind coined zero-quaternol for this number, and is equal to Q<10,4> using quick array notation.