PI
- ABC model of flower development
- Isoelectric point
- P.I. (TV series)
- P1 (disambiguation)
- PI controller
- PL (disambiguation)
- Parental investment
- Partido Independiente
- Partido Intransigente
- Partit per la Independència
- Pass interference
- Paternity Index
- People's Initiative
- Perimeter Institute for Theoretical Physics
- Persistent identifier
- Personal injury
- Philadelphia International
- Philippines
- Phosphatidylinositol
- Pi (disambiguation)
- Piedmont Airlines (1948–89)
- Pirot
- Plasticity index
- Political incorrectness
- Politically Incorrect (blog)
- Polyimide
- Ponderal index
- Poris railway station
- Power integrity
- Preliminary injunction
- Preußische Instruktionen
- Primary immunodeficiency
- Primitive Instinct
- Principal investigator
- Privacy International
- Private investigator
- Pro forma
- Processing Instruction
- Profitability index
- Programmed instruction
- Propidium iodide
- Protease inhibitor (biology)
- Protease inhibitor (pharmacology)
- Provider-independent address space
- Pulsatility index
- Seattle Post-Intelligencer
- Spark-ignition engine