PI
- pi – (s) Pāli language (ISO 639-1 code)
- Pi – (s) Pebi
- PI – (i) PolyImide – PolyIsoprene – Private Investigator
- PIB – (i) PolyIsoButylene
- PIC – (i) Person In Charge
- PICNIC – (a) Problem In Chair Not In Computer
- PICS – (i) Private Investment Capital Subscription
- PID – (i) Pelvic Inflammatory Disease – (p) Plan Identification Number – Positive Identification – Procedural Identification – (i) Procurement Identification Description
- PIDOOMA– (a/i) Pulled It Directly Out Of My Ass
- PIF – (a/i) Partners in Flight
- PIG – (a) Pipeline Inspection Gauge
- PIGS (also PIIGS or PIIGGS) – (a) Portugal, Italy, Greece, Spain; PIIGS if including Ireland; PIIGGS if including the United Kingdom (Great Britain)
- PIM – (a/i) Parameterized Ionospheric Model – Path of Intended Motion – Pediatric Index of Mortality – Personal Information Manager/Management – Presence and Interworking Mobility – Processor-In-Memory – Program Integration Manager – Protocol Independent Multicast
- PIN – (a) Personal Identification Number
- PINF – (i) People In Need Foundation (Czech charitable organisation)
- PIPA – (a/i) Program on International Policy Attitudes
- PIR – (i) Priority Intelligence Requirement
- PIRA – (a) Provisional Irish Republican Army
- PIREPS – (p) Pilot Reports
- PITA – (a) Pain In The Ass
- P.I - Philippines Islands
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