magic number

Short definition from Wiktionary

Noun

1. 1
   The number of neutrons or protons in nuclei which are required to fill the major quantum shells, and thus produce exceptionally stable nuclei: 2, 8, 20, 28, 50, 82 and 126.
2. 2
   A metric used to determine a team's required performance to make the playoffs: The sum of one and the wins by one team and losses by an opponent necessary for the team to ensure the opponent cannot catch it in the standings by the end of the regular season. In Major League Baseball, with a regular season of 162 games, the magic number M is calculated as M = 163 - W₁ - L₂, in which W₁ is the leadin
3. 3
   A metric used to determine a team's required performance to make the playoffs: The lowest such number that assures a team a playoff berth.
4. 4
   A number which is manually entered into source code, rather than defined somewhere as a named constant; especially one which is arbitrary or lacks explanation.
5. 5
   A sentinel value which cannot occur in normal use, typically used as a placeholder (e.g. a fictitious date such as 2000-02-32).
6. 6
   A hash code, such as those used by some webmail servers to keep track of sessions.
7. 7
   A number or pattern in a binary file that identifies the filetype.