| Acronym | Meaning |
| PINS | Portable Isotopic Neutron Spectroscopy |
| PIO | Paired Interaction Orbital |
| PIONA | Paraffins |
| PIPECO | Photoion-photoelectron Coincidence Spectroscopy |
| PIPES | Piperazine-1 |
| PIPICO | Photoion-photoion Coincidence |
| PIPLC | Phosphatidylinositol Phospholipase C |
| PIPN | Pseudo Interpenetrating Polymer Network |
| PIPPE | Particle-induced Prompt Particle Emission |
| PIPS | Passivated Implanted Planar Silicon |
| PIR | Protein Identification Resource |
| PIRANHA | Pesticide And Industrial Chemical Risk Analysis And Hazard Assessment |
| PIRAS | Polarized Infrared Absorption Spectroscopy |
| PIRG | Public Interest Research Group |
| PIRI | Photoinduced Rydberg Ionization |
| PIRL | Polyimide Release Layer |
| PIRR | Polarized IR Reflection |
| PIRSS | Polarization IR Surface Spectroscopy |
| PIRU | Public Information Reference Unit |
| PIS | Penning Ionization Spectroscopy |
| PISA | Pharmaceutical Information Systems Association |
| PISA | Pair Interaction Structureless Approximation |
| PISAM | Photon-induced Scanning Auger Microscope |
| PISCES | Passive In-Situ Concn. Extn. Sampler |
| PISCES | Power Plant Integrated Systems: Chem. Emission Studies |
| PIT | Phase Inversion Temperature |
| PITST | Path Integral Transition State Theory |
| PITT | Partitioning Interwell Tracer Test |
| PIUG | Patent Information Users Group |
| PIUS | Process Inherent Ultimate Safety |
| PIV | Particle Image Velocimetry |
| PIVCDW | Phase Integral Variational Continuum Distorted Wave |
| PIXE | Particle Induced X-ray Emission |
| PIXES | Particle Induced X-ray Emission Spectroscopy |
| PIXET | Proton Induced X-ray Emission Tomography |
| PIXIE | Polar Ionospheric X-ray Imaging Experiment |
| PK | Protein Kinase |
| PKA | Protein Kinase A |
| PKB | Protein Kinase B |
| PKC | Protein Kinase C |
| PKD | Polycystic Kidney Disease |
| PKD | Protein Kinase D |
| PKG | Protein Kinase G |
| PKI | Public Key Infrastructure |
| PKR | Protein Kinase R |
| PKS | Polyketide Synthase |
| PLAD | Pulsed Laser Ablation Deposition |
| PLAP | Pulsed-laser Atom Probe |
| PLC | Programmable Logic Controller |
| PLC | Phospholipase C |