What is the X in X-mas?
Greek letter, from Hebrew, literally "little mouth." As the name of the mathematical constant, from 1841 in English, used in Latin 1748 by Swiss mathematician Leonhart Euler (1707-1783), as an abbreviation of Greek periphereia "periphery." For the meaning "printer's term for mixed type," see pie (3).
n. pl. pis
Symbol π The 16th letter of the Greek alphabet.
The pH value for the isoelectric point of a given substance in solution.