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Perfluorohexanoic acid
Perfluorohexanoic acid (PFHxA) is a persistent, colorless liquid PFAS compound used as a short-chain substitute for PFOA, also forming as a degradation product of long-chain PFAS, and restricted in the EU in 2024.
Perfluorononanoic acid
Perfluorononanoic acid (PFNA) is a man-made perfluoroalkyl carboxylic acid used since 1975, mainly as its ammonium salt for fluoropolymer dispersions, and it persists in the environment.
Perfluorooctane
Perfluorooctane is a chemically inert, dense fluorocarbon liquid with the formula C8F18, made by electrochemical fluorination and used in vitreoretinal surgery and Fluorinert FC-77 coolant.
Perfluorooctanesulfonic acid
Perfluorooctanesulfonic acid, known as PFOS, is a synthetic fluorinated surfactant once used in stain repellents and firefighting foams, now regulated worldwide as a persistent organic pollutant.
Perfluorooctanoic acid
Perfluorooctanoic acid (PFOA) is a synthetic PFAS "forever chemical" made since 1947, classified carcinogenic to humans in 2023 and limited in US drinking water to 4 parts per trillion in 2024.
Perfluoropolyether
Perfluoropolyether (PFPE) is a synthetic fluorinated polymer whose oils and greases are chemically inert, non-toxic, and nonflammable, used as lubricants in spacecraft, hard disk drives, and vacuum pumps since the 1960s.
Perfluoropropane
Perfluoropropane, also called octafluoropropane (C₃F₈), is a colorless, odorless, nonflammable gas used as an intraocular gas in retinal detachment surgery and in ultrasound contrast agents.
Performance
Performance is the degree to which an organization, program, or system achieves its stated goals, measured by frameworks such as the Balanced Scorecard, introduced by Kaplan and Norton in 1992.
Performance appraisal
Performance appraisal, also known as a performance review or evaluation, is the formal process of judging an employee's job performance against set standards, usually producing a written rating of record.
Performance indicator
A performance indicator, also called a performance metric, is a measurable variable representing the status of operations, processes, or outcomes. Indicators used by top management are called key performance indicators, or KPIs.
Pericyclic reaction
A pericyclic reaction, or pericyclic process, is an organic reaction in which bonds reorganize in a single concerted step through a cyclic transition state, with no intermediates.
Peridot
Peridot, also known as chrysolith, is the gem-quality yellowish-green variety of the mineral olivine, prized for its intrinsic iron-derived color and found in lavas and meteorites.
Peridotite
Peridotite is a coarse-grained ultramafic igneous rock rich in olivine, making up most of the Earth's upper mantle and reaching the surface in xenoliths and ophiolites.
Peridynamics
Peridynamics is a nonlocal continuum theory and simulation method introduced by Stewart A. Silling in 2000, in which fracture emerges from bond breaking rather than a separate crack-tracking algorithm.
Perihelion precession of Mercury
The perihelion precession of Mercury is the gradual rotation of the planet's closest point to the Sun; about 43 arcseconds per century remained unexplained until Einstein's 1915 general relativity.
Perillaldehyde
Perillaldehyde, also called perillyl aldehyde, is a natural monoterpene aldehyde that gives perilla (shiso) leaves and citrus peel their characteristic pungent aroma and is used as a flavoring.
Perimeter
Perimeter is the total length of the boundary of a closed geometric figure; for a polygon it is the sum of side lengths, and for a circle it is the circumference.
Perimeter Institute for Theoretical Physics
The Perimeter Institute for Theoretical Physics is an independent research institute in Waterloo, Ontario, Canada, founded in 1999 with a $100 million gift from BlackBerry co-founder Mike Lazaridis.
Perimeter of an ellipse
The perimeter of an ellipse is the total length of its boundary, given exactly by 4a times the complete elliptic integral and usually computed with Ramanujan's 1914 approximation formulas.
Period (periodic table)
A period is a horizontal row of the periodic table; there are seven, containing 2, 8, 8, 18, 18, 32, and 32 elements, each ending at a noble gas.
Period 2 element
A period 2 element is one of the eight chemical elements in the second row of the periodic table: lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and neon.
Period 3 element
A Period 3 element is one of the eight chemical elements in the third row of the periodic table, sodium through argon, atomic numbers 11 to 18.
Period 7 element
A period 7 element is one of the 32 chemical elements in the seventh row of the periodic table, from francium (87) to oganesson (118), all of them radioactive.
Period 8 element
A period 8 element is a hypothetical chemical element in the eighth row of the extended periodic table, starting with element 119, which no laboratory has yet synthesized.
Period mapping
A period mapping assigns to each point in the base of a family of algebraic varieties the cohomology of the fiber, viewed as a point in a moduli space of Hodge structures.
Period-doubling bifurcation
A period-doubling bifurcation, also called a flip bifurcation, is a local bifurcation where a periodic orbit loses stability and a new orbit of twice the period appears.
Period-luminosity relation
The period-luminosity relation, properly Leavitt's law, is the empirical rule discovered by Henrietta Leavitt in 1908 linking Cepheid variables' pulsation periods to their luminosity, anchoring the cosmic distance ladder.
Periodate
Periodate is a monovalent inorganic anion, IO4⁻, with iodine in the +7 oxidation state, best known as the reagent in the Malaprade reaction that cleaves vicinal diols.
Periodate oxidation
Periodate oxidation, known as the Malaprade reaction, cleaves vicinal diols with periodate in water to give carbonyl fragments, and is used for carbohydrate linkage analysis and biomaterials.
Periodic boundary conditions
Periodic boundary conditions (PBC) are a simulation technique in molecular dynamics and Monte Carlo methods where a particle leaving the box re-enters through the opposite face.