(physics) The only stable isotope of phosphorus, ³¹₁₅P, having fifteen protons and sixteen neutrons

A nonexistent date of the Gregorian calendar, especially in context of having scheduled events that will never occur.

Abbreviation of thirty-first.

(often with "the") Detroit, after the 313 area code.

(often with "the") Chicago, after the 312 area code.

(Canada, US, telephony) The telephonic service for accessing some local government services, depending on jurisdiction