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- Paal–Knorr synthesis – Carl Paal and Ludwig Knorr
- Pareto chart, distribution, efficiency, index, principle – Vilfredo Federico Damaso Pareto
- Pareto–Zipf law (a.k.a. Zipf–Mandelbrot law) – Vilfredo Pareto and George K. Zipf (or Benoît Mandelbrot)
- Parrondo's games, paradox – Juan Manuel Rodríguez Parrondo
- Paschen curve, line, law – Friedrich Paschen
- Paschen–Back effect – Friedrich Paschen and Ernst Back
- Paternò–Büchi reaction – Emanuele Paternò and George Hermann Büchi
- Pasteur effect – Louis Pasteur
- Pauli exclusion principle – Wolfgang Pauli
- Peano curve – Giuseppe Peano
- Pearson–Anson effect – Stephen Oswald Pearson and Horatio Saint George Anson
- Péclet number – Jean Claude Eugène Péclet
- Peltier effect – Jean Charles Athanase Peltier
- Perlin noise – Ken Perlin
- Perron–Frobenius theorem – Oskar Perron, and Ferdinand Georg Frobenius
- Petkau effect – Abram Petkau
- Petri dish – Julius Richard Petri
- Petri net – Carl Adam Petri
- Peyer's patches – Johann Conrad Peyer
- Pfeiffer effect – Paul Pfeiffer
- Pfund line/series – August Herman Pfund
- Phillips curve – William Phillips (economist)
- Pigou effect – Arthur Cecil Pigou
- Pisot–Vijayaraghavan number – Charles Pisot and Tirukkannapuram Vijayaraghavan
- Planck constant, length, mass, time – Max Planck
- Platonic year – Plato
- Pockels effect – Friedrich Carl Alwin Pockels
- Pogson ratio – Norman Robert Pogson
- Poincaré map, section – Jules-Henri Poincaré
- Poincaré–Bendixson theorem – Jules-Henri Poincaré and Ivar Otto Bendixson
- Poinsot's spirals – Louis Poinsot
- Polchinski's paradox – Joseph Polchinski
- Polian vesicles – Giuseppe Saverio Poli
- Potts cluster, Potts model (a.k.a. Ashkin-Teller model) – Renfrey B. Potts
- Pourbaix diagram – Marcel Pourbaix
- Poynting effect, vector – John Henry Poynting
- Poynting-Robertson effect – John Henry Poynting and Howard Percy Robertson
- Prandtl number – Ludwig Prandtl
- Primakov effect – ? Primakov
- Proteus phenomenon – Proteus (mythological god)
- Prouho's membrane – Henri Prouho
- Pulfrich effect – Carl Pulfrich
- Purkinje effect/shift – Johannes Evangelista Purkinje
- Pygmalion effect (a.k.a. Rosenthal effect, Observer-expectancy effect) – Pygmalion (and Robert Rosenthal)
Read more about this topic: Scientific Phenomena Named After People