Physical world and mathematics

Pleiades Moving Group

The Pleiades Moving Group, also called the Local Association, is an unbound stream of young stars, 20 to 150 million years old, sharing the Pleiades cluster's motion.

Pleistocene

The Pleistocene, often called the Ice Age, is the geological epoch from 2.58 million to 11,700 years ago, spanning Earth's most recent repeated glaciations before the Holocene.

Pleochroism is the selective absorption in crystals of light vibrating in different planes, so colored, doubly refracting crystals show different colors from different directions, as in iolite and tanzanite.

Plerixafor

Plerixafor (trade name Mozobil, originally AMD3100) is a CXCR4 antagonist drug used with filgrastim to mobilize hematopoietic stem cells for transplantation in non-Hodgkin's lymphoma and multiple myeloma.

Plethysm

Plethysm is an operation on symmetric functions, written f[g], in which one symmetric function is substituted into another, introduced by D. E. Littlewood in 1936.

Plexcitons

Plexcitons, short for plasmon–exciton polaritons, are hybrid quasiparticles formed when a plasmon in a metallic nanostructure couples to an exciton, forming two new states under strong coupling.

Pliers

Pliers are hand tools with two handles, a pivot, and jaws, used to grip, bend, twist, or cut objects with more force than the hand alone.

Plinian eruption

A Plinian eruption, also called a Vesuvian eruption, is the most powerful class of explosive volcanic eruption, named for Pliny the Younger's account of Vesuvius in A.D. 79.

Pliocene

The Pliocene Epoch spans 5.333 to 2.58 million years ago, following the Miocene and preceding the Pleistocene, with warm early climates and growing Northern Hemisphere ice sheets.

Plücker coordinates

Plücker coordinates are the homogeneous coordinates of a subspace in projective geometry: the maximal minors of a matrix spanning it, first considered by Julius Plücker in 1869.

Plug flow reactor

A plug flow reactor (PFR), also called a tubular reactor, is a chemical reactor where reactants flow through a pipe as unmixed plugs, one of two ideal reactor models.

Plum pudding model

The plum pudding model, also known as the Thomson model, was J. J. Thomson's 1904 model of the atom as a sphere of positive charge containing electrons.

Plumb bob

A plumb bob, also called a plumb-line, is a weighted metal point hung on a string that gives builders and surveyors a vertical reference line.

Plumbagin

Plumbagin, chemically 5-hydroxy-2-methylnaphthalene-1,4-dione, is a naturally occurring naphthoquinone from the roots of the medicinal plant Plumbago zeylanica, studied for anticancer, antimicrobial, and anti-inflammatory activities but limited by toxicity.

PLUMED

PLUMED, the community-developed PLUgin for MolEcular Dynamics, is an open-source library for free-energy calculations and enhanced sampling in molecular dynamics, interfaced with codes such as GROMACS, NAMD, and LAMMPS.

Plünnecke–Ruzsa inequality

The Plünnecke–Ruzsa inequality is a theorem in additive combinatorics stating that if |A+B| ≤ K|A|, then |kB − ℓB| ≤ K^(k+ℓ)|A|, proved by Plünnecke in 1970 and Ruzsa in 1989.

Plus and minus signs

The plus sign (+) and minus sign (−) are mathematical symbols for addition and subtraction, first printed in 1489 in Johannes Widmann's Leipzig arithmetic book.

Plus–minus sign

The plus or minus symbol (±) is a mathematical sign indicating a value may be either added or subtracted, also used to report measurement uncertainty.

Plutino

A plutino is a trans-Neptunian object locked in a 3:2 resonance with Neptune, orbiting the Sun twice for every three Neptune orbits; Pluto is the largest known member.

Pluto

Pluto is a dwarf planet in the Kuiper Belt beyond Neptune, reclassified from ninth planet in 2006 and explored by NASA's New Horizons flyby of 2015.

Plutonium

Plutonium is a radioactive, silvery-gray actinide metal with atomic number 94, first isolated at Berkeley in 1940, used in nuclear weapons, recycled reactor fuel, and spacecraft power sources.

Plutonium dioxide

Plutonium dioxide (PuO₂), also called plutonium(IV) dioxide, is the tetravalent oxide of plutonium, a refractory ceramic used as MOX fuel and in spacecraft power systems.

Plutonium-238

Plutonium-238 is a radioactive isotope of plutonium with an 87.7-year half-life that produces about 0.567 watts of heat per gram, used to power NASA spacecraft.

Plutonium-239

Plutonium-239 (Pu-239) is a fissile isotope of plutonium with a half-life of 24,110 years, produced from uranium-238 in nuclear reactors and used as the main explosive ingredient of nuclear weapons.

Plutonium-240

Plutonium-240 is a radioactive isotope of plutonium with a half-life of about 6,560 years, formed in reactors when plutonium-239 captures a neutron; its high spontaneous fission rate shaped early bomb design.

Plutonium(VII) compounds

Plutonium(VII) compounds contain plutonium in the +7 oxidation state, the highest confirmed state of the element, known almost entirely from short-lived alkaline solutions that are dark green.

Plymouth Marine Laboratory

Plymouth Marine Laboratory (PML) is an independent marine research charity in Plymouth, England, formed in 1988 by NERC, known for ocean acidification research and long-term ocean monitoring since 1903.

PM

PM, from the Latin post meridiem meaning after midday, is the 12-hour clock designation for the half of the day from noon to midnight, so 12:00 PM is noon.

PM6

PM6 is a fluorinated donor-acceptor copolymer used as the electron donor in organic solar cells. Its 2019 pairing with the acceptor Y6 reached 15.7% efficiency and became a benchmark.