magic number

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of the numbers, 2, 8, 20, 28, 50, 82, or 126, that represent the number of neutrons or protons in strongly bound and exceptionally stable atomic nuclei.
• noun Sports The number of wins by a leading team and losses by its closest competitor required in order for the leading team to capture a championship or secure a playoff spot.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics 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 & 126
• noun baseball A metric used to determine a team's required performance to make the playoffs.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the atomic number of an extra stable strongly bound atomic nucleus: 2, 8, 20, 28, 50, 82 or 126