42,772 articles
Ordinal data
Ordinal data are categorical data whose values can be logically ordered or ranked, such as grades, clothing sizes, and Likert responses, while the spacing between categories is unknown or unequal.
Ordinal notation
An ordinal notation is a formal system of finite expressions with a computable ordering that names ordinals, central to ordinal analysis in proof theory and to proving program termination.
Ordinal number
An ordinal number (or ordinal) is the order type of a well-ordered set, introduced by Georg Cantor in 1883 to extend natural numbers to infinite positions.
Ordinary differential equation
An ordinary differential equation (ODE) is an equation involving an unknown function of one variable and its derivatives, used to model motion, circuits, chemical reactions, and populations.
Ordinary generating function
An ordinary generating function (OGF) encodes a sequence as a power series whose coefficients are the sequence's terms, distinguishing it from weighted variants such as exponential generating functions.
Ordinary least squares
Ordinary least squares (OLS) regression is the standard estimator of the linear regression model, choosing coefficients that minimize the sum of squared residuals, described as the workhorse of applied econometrics.
Ordinary Time
Ordinary Time (Latin: Tempus per annum) is the longest season of the Roman Catholic liturgical year, 33 or 34 weeks outside Advent, Christmas, Lent, and Easter, with green vestments.
Ordos Basin
The Ordos Basin is a large sedimentary basin in the western North China Block, China, with a depositional history from the Mesoproterozoic to the Quaternary.
Ordovician
Ordovician Period, second Paleozoic period, 485.4–443.8 Ma (ICS), named by Lapworth; seven GSSP-defined stages, Great Ordovician Biodiversification Event, high CO2 with cooling, and the Hirnantian glaciation and mass extinction.
Ordovician–Silurian glaciation
The Ordovician–Silurian glaciation, often called the Hirnantian glaciation, was a brief, severe ice age about 445 million years ago that helped drive a major mass extinction.
Ore
Ore is naturally occurring rock or mineralized material that contains a valuable substance in sufficient concentration, and is accessible enough, to be mined and processed at a profit.
Ore microscopy and automated mineralogy
Ore microscopy examines polished ore sections under a microscope to identify minerals and textures; automated mineralogy performs the same characterization quantitatively using SEM-based imaging and X-ray analysis.
Oregon Vortex
The Oregon Vortex, also called the House of Mystery, is a Gold Hill, Oregon roadside attraction opened in 1930 by John Litster, the earliest documented US mystery spot, explained scientifically as optical illusions.
Orelabrutinib
Orelabrutinib (ICP-022, 宜諾凱), InnoCare Pharma's oral irreversible highly selective BTK inhibitor, approved in China for r/r CLL/SLL, r/r MCL, r/r MZL, and first-line CLL/SLL, also in development for SLE and multiple sclerosis.
Oren Scherman
Oren A. Scherman is a professor of supramolecular and polymer chemistry at the University of Cambridge, known for self-healing cucurbituril-based materials and directing the Melville Laboratory for Polymer Synthesis.
Oreste Piccioni
Oreste Piccioni (1915–2002) was an Italian experimental particle physicist, the last of Enrico Fermi's students, who co-discovered the antineutron in 1956 and long disputed his exclusion from the 1955 antiproton discovery.
Organ-on-a-chip
An organ-on-a-chip is a microfluidic device that cultures living human cells in tiny channels to model organ function, developed since 2010 to predict drug toxicity better than animal models.
Organic acid
An organic acid is an organic compound with acidic properties, most commonly a carboxylic acid with the structure RC(=O)OH, and includes sulfonic acids and some phenols.
Organic acid anhydride
An organic acid anhydride is a compound with two acyl groups bonded to one oxygen atom, formed by removing water from two carboxylic acid molecules.
Organic chemistry
Organic chemistry is the study of carbon compounds, which form chains and rings and make up almost all of the more than 197 million known chemical compounds.
Organic color centers in carbon nanotubes
Organic color centers (OCCs) are photon-emitting quantum defects created by bonding organic functional groups to semiconducting carbon nanotubes, trapping excitons and emitting single shortwave-infrared photons at room temperature.
Organic compound
An organic compound is a chemical compound containing carbon, often bonded to oxygen or nitrogen. Historically related to living things, it is now defined by the presence of carbon.
Organic diselenides and ditellurides
Organic diselenides and ditellurides are organochalcogen compounds containing a covalent bond between two selenium or two tellurium atoms, serving as stable precursors to electrophilic, nucleophilic, and radical selenium reagents.
Organic electrochemistry
Organic electrochemistry is the branch of synthetic chemistry that uses electron transfer at electrode surfaces, rather than stoichiometric chemical reagents, to oxidize or reduce organic molecules and form new bonds.
Organic laser
An organic laser is a laser whose gain medium is an organic dye or conjugated semiconductor, chemically tunable from ultraviolet to near-infrared and typically optically pumped.
Organic Letters
Organic Letters, also known as Org. Lett., is a peer-reviewed American Chemical Society journal for short, rapid communications in organic chemistry, founded in 1999 and led since 2021 by Marisa C. Kozlowski.
Organic matter
Organic matter is carbon-based material derived from living organisms or lab synthesis. In environmental science, it refers to natural residues in soils, sediments, and water.
Organic photoredox catalysis
Organic photoredox catalysis is metal-free visible-light-driven catalysis in which organic dyes, such as eosin Y, replace ruthenium and iridium complexes to generate reactive radical intermediates under mild conditions.
Organic photoredox catalyst
An organic photoredox catalyst is a metal-free organic compound that absorbs visible light and uses its excited state to mediate electron- or atom-transfer reactions without metals.
Organic polariton condensates
An organic polariton condensate is a macroscopically occupied quantum state of exciton-polaritons, hybrid light-matter quasiparticles that emit coherent light, first demonstrated in anthracene in 2010.