See also: Appendix:Variations of "pi"
- Initialism of partial induction (see AI)
- Initialism of personal injury.
- Initialism of politically incorrect (see PC)
- Initialism of principal investigator (lead researcher on a grant-funded project)
- Initialism of Prison Industries (the prison-run work program for inmates)
- Initialism of private investigator.
- Initialism of pseudoincest.
- (biochemistry) Abbreviation of phosphatidylinositol (a phospholipid component of eukaryotic cell membranes, also abbreviated in PtdIns)
- (electronics) Initialism of power integrity.
- (informatics, SGML, XML) Initialism of Processing Instruction.
- (organic chemistry) Initialism of polyimide.
- (colloquial, Philippines) Initialism of the Philippines (PI standing for Philippine Islands)
- We're going back to the PI for a vacation this summer.
- Abbreviation of prohibited immigrant.