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Two-dimensional nuclear magnetic resonance spectroscopy
Two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy correlates NMR signals along two frequency axes, resolving overlapping peaks and revealing which nuclei are bonded or close in space.
Two-dimensional polyaniline
Two-dimensional polyaniline (2D PANI) is an atomically thin crystalline framework built from polyaniline chemistry, first synthesized in 2016 and reported as the metallic multilayer crystal 2DPANI in 2025.
Two-dimensional polyimide
Two-dimensional polyimide (2DPI) is a one-monomer-unit-thick polymer sheet with repeat units covalently linked by imide bonds in two directions, first synthesized in crystalline form in 2019.
Two-dimensional polymer
Two-dimensional polymers, or 2DPs, are covalently bonded molecular sheets one monomer unit thick with long-range periodic order, first synthesized in 2012 and studied for separation and electronics.
Two-dimensional quantum turbulence
Two-dimensional quantum turbulence is the chaotic flow of a quantum fluid, such as a superfluid, confined to two dimensions, where all rotation is carried by quantized vortices.
Two-dimensional space
Two-dimensional space, also called bi-dimensional space, is a mathematical space in which every point is specified by two coordinates, the standard example being the Euclidean plane of elementary geometry.
Two-dimensional superconductivity
Two-dimensional superconductivity is superconductivity confined to a single atomic layer or interface electron gas, seen in systems like monolayer Pb on silicon and monolayer FeSe on SrTiO₃.
Two-dimensional warranty models
A two-dimensional warranty model covers a product until an age limit or a usage limit is reached, whichever comes first, as in 5 years or 40,000 miles.
Two-dog Palette
The Two-dog Palette, also called the Ashmolean Palette, is a late Predynastic Egyptian ceremonial cosmetic palette excavated at Hierakonpolis and held in the Ashmolean Museum, Oxford.
Two-domain system
Two-domain system: classification of life into Bacteria and Archaea, with eukaryotes nested within Archaea; also called the eocyte-style 2D tree, supported by Asgard archaea phylogenomics and contested by three-domain proponents.
Two-electrode voltage clamp
Two-electrode voltage clamp (TEVC) is an electrophysiology technique that uses two microelectrodes to hold a large cell's membrane voltage and measure currents through ion channels, the standard method for recording expressed channels in Xenopus laevis oocytes.
Two-electron oxygen reduction to hydrogen peroxide
Electrocatalytic two-electron oxygen reduction to hydrogen peroxide (2e⁻ ORR) is an electrochemical reaction that reduces oxygen at a catalyst surface to form hydrogen peroxide instead of water.
Two-Face
Two-Face is a DC Comics Batman supervillain, also known as Harvey Dent (originally Harvey "Apollo" Kent), Gotham's district attorney scarred by acid and ruled by coin flips; first appearance Detective Comics #66 (1942), with a 2024 solo series.
Two-factor theory
Herzberg's two-factor theory (motivation-hygiene theory), from The Motivation to Work (1959): motivators create satisfaction, hygiene factors only prevent dissatisfaction, as independent sets.
Two-fluid model
The two-fluid model describes superfluid helium II as a mixture of a frictionless superfluid and a viscous normal fluid, proposed by Laszlo Tisza in 1938.
Two-gap model
The two-gap model is a development-economics growth model in which a developing country's growth rate is set by whichever is larger: the savings gap or the foreign-exchange gap.
Two-graph
A two-graph is a combinatorial structure of vertices and triples of them such that every set of four vertices contains an even number of selected triples.
Two-Gun Cohen
Two-Gun Cohen (Morris Abraham Cohen, known in Chinese as Ma Kun) was a Jewish adventurer who served Sun Yat-sen as bodyguard and became a general in the Chinese National Revolutionary Army.
Two-Higgs-doublet model
The two-Higgs-doublet model (2HDM) is one of the simplest extensions of the Standard Model, adding a second Higgs doublet to yield five physical Higgs bosons instead of one.
Two-hour marathon
Two-hour marathon: running 42.195 km under 2:00 at ~2:50/km. Achieved unofficially by Kipchoge (1:59:40, Vienna 2019) and officially by Sawe (London); official record Kiptum 2:00:35.
Two-hybrid screening
Two-hybrid screening (yeast two-hybrid, Y2H) is a genetic assay, introduced by Fields and Song in 1989, that detects protein-protein interactions by reconstituting a transcription factor from DNA-binding and activation domain fusions, with known high false-positive and false-negative rates.
Two-Lane Blacktop
Two-Lane Blacktop is Monte Hellman's 1971 Universal road film starring James Taylor, Dennis Wilson, Laurie Bird, and Warren Oates in a cross-country race for car pink slips; a box-office flop later canonized in the National Film Registry (2012) and released by Criterion.
Two-legged tie
Two-legged tie: a football knockout pairing played over two matches, one home and one away, decided on aggregate. Formerly broken by the away goals rule (1965–2021), now by extra time and penalties; second-leg home advantage is debated.
Two-level system defects
Two-level system defects (TLS defects) are atomic-scale defects in superconducting quantum circuits that behave as quantum two-level systems, and the dominant source of decoherence and noise in superconducting qubits.
Two-level utilitarianism
Two-level utilitarianism, R.M. Hare's theory from Moral Thinking (1981), separates critical-level act-utilitarian reasoning (archangels) from intuitive-level everyday principles (proles), answering rule-worship and counterintuitive-case objections.
Two-line element set
A two-line element set (TLE) is a standardized two-line, 69-character format describing the mean orbital elements of a cataloged Earth-orbiting object, used with the SGP4 or SDP4 models.
Two-link flexible manipulator
A two-link flexible manipulator is a robotic arm with two light links that bend during motion, whose control must combine trajectory tracking with vibration suppression.
Two-man rule
The two-man rule, in nuclear weapons practice called the two-person concept (TPC), requires at least two authorized people to be present for critical tasks so no one can act alone.
Two-mode squeezed vacuum
The two-mode squeezed vacuum, also called the twin-beam state, is an entangled state of two optical modes in which the modes carry perfectly correlated photon numbers.
Two-nation theory
Two-nation theory (دو قومی نظریہ, do qaumī nazariya): the doctrine that Hindus and Muslims of the subcontinent are two separate nations, adopted by the Muslim League in the 1940 Lahore Resolution, justifying the 1947 partition of India and challenged by Bangladesh's 1971 secession.