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Paksha
Paksha (पक्ष) is a lunar fortnight in the Hindu calendar, half of a lunar month: the bright shukla paksha when the Moon waxes, and the dark krishna paksha when it wanes.
Pál Turán
Pál Turán (1910–1976) was a Hungarian mathematician who founded extremal graph theory with his 1940 theorem, created the power sum method, and collaborated with Erdős.
Palaeoclimate Modelling Intercomparison Project
The Palaeoclimate Modelling Intercomparison Project (PMIP) coordinates standardized simulations of past climates in many climate models, testing them against palaeoclimate reconstructions since the early 1990s.
Palatini formulation
The Palatini formulation is a variational approach to general relativity in which the metric and affine connection are varied independently; it is used mainly in modified gravity theories.
Pale Blue Dot
Pale Blue Dot: Voyager 1 photograph of Earth taken 14 February 1990 from 6 billion km, showing Earth as a 0.12-pixel crescent in scattered sunlight; part of the 60-frame Family Portrait, named after Carl Sagan's 1994 book.
Paleoarchean
The Paleoarchean is the second era of the Archean Eon, spanning 3,600 to 3,200 million years ago, defined by fixed ages and hosting many of the oldest claimed records of life.
Paleocene
The Paleocene is the first epoch of the Paleogene Period, lasting from 66 to 56 million years ago, when mammals diversified after the extinction that ended the dinosaurs.
Paleocene–Eocene Thermal Maximum
The Paleocene–Eocene Thermal Maximum, or PETM, was a geologically rapid global warming and carbon release event about 56 million years ago, at the Paleocene–Eocene boundary.
Paleoclimate proxy
A paleoclimate proxy is a measurable property of a natural archive, such as ice cores, tree rings, or corals, that stands in for climate measurements made before instruments existed.
Paleoclimatology
Paleoclimatology, the study of Earth's ancient climate history, reconstructs conditions before instrumental records by reading natural archives such as tree rings, corals, ice cores, and sediments.
Paleogene
The Paleogene Period is the first period of the Cenozoic Era, spanning 66 to 23 million years ago and divided into the Paleocene, Eocene, and Oligocene epochs.
Paleogeography of the India–Asia collision system
The paleogeography of the India–Asia collision system reconstructs the continents and oceans that preceded India's collision with Asia, which closed the Neo-Tethys Ocean and built the Himalayas.
Paleointensity
Paleointensity is the study of Earth's ancient magnetic field strength, recovered from rocks and fired clays by comparing their natural magnetization with one induced in a known laboratory field.
Paleolimnology
Paleolimnology is the study of lake and wetland sediments, using physical, chemical, and biological proxies in dated sediment cores to reconstruct past lake, catchment, and climate conditions over decades to millennia.
Paleomagnetism
Paleomagnetism is the study of the Earth's past magnetic field, recorded in rocks, sediments, and fired materials; it underpins the geomagnetic polarity timescale and continental reconstructions.
Paleosol
A paleosol is a fossil soil that formed on a landscape of the past and is preserved in the geological record; its study is called paleopedology, and buried soils define geosols in stratigraphy.
Paleostress analysis
Paleostress analysis is a branch of structural geology that reconstructs past stress fields from deformation structures, most commonly fault-slip data and calcite twins, using inversion methods developed since the 1970s.
Paleozoic
The Paleozoic Era, spanning 541 to 251.9 million years ago, saw fish diversify and life colonize land, ending with the greatest mass extinction in Earth's history.
Palindromic number
A palindromic number, also called a numeral palindrome, reads the same forwards and backwards in some base, such as 121 or 94200249 in decimal.
Palisades Fire
The Palisades Fire, a wind-driven wildfire, ignited on January 7, 2025 in Pacific Palisades, Los Angeles, burning 23,448 acres, destroying 6,845 structures, and killing 12 civilians.
Pallab K. Bhattacharya
Pallab K. Bhattacharya, born in India in 1949, is an electrical engineer at the University of Michigan known for discovering self-organized quantum dots and commercializing quantum dot lasers.
Palladium
Palladium (Pd, atomic number 46) is a silvery-white transition metal of the platinum group, discovered by William Hyde Wollaston in 1803 and best known as the active metal in automotive catalytic converters.
Palladium catalysis
Palladium catalysis is the use of palladium complexes to accelerate organic reactions, most prominently cross-couplings that join two carbon-containing fragments, accounting for roughly a quarter of reactions in the pharmaceutical industry.
Palladium hydride in electrocatalysis
Palladium hydride (PdHx) is the phase formed when hydrogen occupies interstitial sites in palladium's lattice, the reactive state palladium electrodes adopt under cathodic potentials in reactions from CO2 reduction to hydrogen evolution.
Palladium on carbon
Palladium on carbon (Pd/C, also called palladium on charcoal) is a heterogeneous catalyst of palladium particles on activated carbon, used widely for hydrogenation and cross-coupling reactions in organic synthesis.
Palladium-catalyzed aryl fluorination
Palladium-catalyzed aryl fluorination is a set of methods that form aryl carbon–fluorine bonds using palladium catalysts, replacing multistep classical routes with direct conversion of aryl triflates, aryl halides, or arene C–H bonds into aryl fluorides.
Palladium-catalyzed cross-coupling
Palladium-catalyzed cross-coupling is a class of reactions that forms carbon–carbon bonds between organic fragments, underlying roughly a quarter of pharmaceutical-industry reactions and the 2010 Nobel Prize in Chemistry.
Palladium(II) chloride
Palladium(II) chloride, also called palladium dichloride (PdCl2), is a rust-colored to dark brown solid that is the common starting material in palladium chemistry and the palladium component of the Wacker oxidation catalyst.
Palladium/norbornene cooperative catalysis
Palladium/norbornene cooperative catalysis, known as the Catellani reaction, installs an electrophile at the ortho position and couples a nucleophile at the ipso position of aryl halides in one operation.
Pallene (moon)
Pallene, designated Saturn XXXIII, is a small moon of Saturn about 4 kilometers across, discovered by the Cassini imaging team in 2004 and orbited by a faint dusty ring.