magic number
noun
1
: one of a set of numbers for which an atomic nucleus exhibits a high degree of stability when either the proton or neutron count is equal to the number
2
: a number that represents a combination of wins for a leader (such as in a baseball pennant race) and losses for a contender which mathematically guarantees the leader's winning the championship
Word History
First Known Use
1947, in the meaning defined at sense 2