a set in the polynomial hierarchy (Pi function), the Gamma function when offset to coincide with the factorial , the complete elliptic integral of the third kind , the th homotopy group of -calculus, a process calculus Prime-counting function or , the population proportion in statistics Physics Pion or π, a subatomic particle Other uses Pi (prefix symbol), or pebi, in computing Π, for plaintiff, in legal shorthand Π, stands for Pierikos F.C. kit logo Pi Day, March 14 (3/14 in the US) Principal investigator, a lead researcher Private investigator See also PI (disambiguation) P (disambiguation) Pai (disambiguation) Pie (disambiguation) Pye (disambiguation)