magic number

noun

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. A metric used to determine a team's required performance to make the playoffs.

3. 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.

   We replaced the magic number 31 with a constant called MAX_DAYS_IN_MONTH.

4. A number which cannot occur in normal use, typically used as a placeholder (e.g. a fictitious date such as 2000-02-32).

5. A hash code, such as those used by some webmail servers to keep track of sessions.

6. A number in a file’s content that identifies the filetype.