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Two-neutron emission
Two-neutron emission (2n emission) is a radioactive decay mode in which a nucleus beyond the neutron drip line expels two neutrons at the same time.
Two-neutron radioactivity
Two-neutron radioactivity is a proposed exotic decay mode in which a nucleus beyond the neutron drip line emits two neutrons simultaneously as a genuine three-body process with a measurable lifetime.
Two-part episode
Two-part episode: a single TV story split across two installments, usually with a cliffhanger break; also called a two-parter or multi-part episode; scheduled over two nights to retain viewers and counted as one or two episodes depending on cataloging rules.
Two-party system
Two-party system: a political arrangement where two major parties, such as the US Democrats (1824) and Republicans (1854), dominate elections; sustained by winner-take-all plurality rules, unlike proportional multiparty systems.
Two-party-preferred vote
Two-party-preferred (TPP) vote: Australian House of Representatives statistic distributing all preferences between the ALP and the Coalition, distinct from two-candidate preferred, used for swing and government-formation prediction.
Two-phase commit protocol
The two-phase commit protocol (2PC) is an atomic commitment protocol for distributed transactions in which a coordinator asks all participants to prepare and commits only if every one agrees.
Two-phase electric power
Two-phase electric power, or two-phase power, is an AC polyphase system carrying two equal voltages 90 degrees apart, historically over four wires, displaced by three-phase for transmission.
Two-phase immersion cooling
Two-phase immersion cooling is a data center cooling method in which servers are submerged in a dielectric fluid that boils at a low temperature, condensing and recycling the vapor.
Two-phase locking
Two-phase locking (2PL) is a concurrency control protocol for database transactions in which a transaction acquires all its locks before releasing any, guaranteeing that the execution is serializable.
Two-photon absorption
Two-photon absorption (TPA) is a nonlinear optical process where a molecule absorbs two photons at once, predicted in 1931 and first observed with lasers in 1961.
Two-photon emission
Two-photon emission is a process in which hydrogen in its metastable 2s state decays to 1s by emitting two photons, producing a continuous ultraviolet spectrum.
Two-photon excitation microscopy
Two-photon excitation microscopy (2PE, two-photon laser scanning microscopy): fluorescence imaging using simultaneous two-photon near-infrared absorption, invented by Denk, Strickler, and Webb in 1990; enables deep, low-photodamage in vivo tissue imaging.
Two-photon lithography
Two-photon lithography is a mask-free additive microfabrication method that uses a pulsed laser to polymerize resin, building 3D microstructures with features down to tens of nanometers.
Two-point calibration
Two-point calibration is a metrology method that adjusts an instrument's offset and span using measurements at two known reference points, valid only when the response is linear.
Two-point conversion
Two-point conversion: post-touchdown run or pass play worth two points. Adopted by the NCAA in 1958, the AFL in 1960, and the NFL in 1994; NFL conversion rate about 47 percent.
Two-point discrimination
Two-point discrimination (2PD), from Weber's 1834 method, measures the minimum skin separation at which two blunt contacts feel distinct; thresholds range from 1–2 mm at the tongue tip to 20–30 mm on the hand back, but nonspatial cues inflate performance, prompting the two-point orientation discrimination alternative.
Two-port network
A two-port network, or two port, is an electrical network with four terminals grouped into two pairs, described by a 2 × 2 parameter matrix.
Two-proportion Z-test
The two-proportion z-test, also called the two-sample proportion z-test, is a statistical hypothesis test comparing success rates between two independent groups, used in clinical trials and A/B testing.
Two-proton emission
Two-proton emission is a rare form of radioactivity in which an extremely proton-rich nucleus sheds two protons at once, predicted in 1960 and first observed in 2002 in iron-45.
Two-round system
Two-round system (TRS, runoff election): a two-stage voting method used in France, Brazil, and most direct presidential elections, requiring 50%+1; a top-two runoff decides when no candidate wins a first-round majority.
Two-seam fastball
Two-seam fastball (sinker, two-seamer): fastball gripped along the two close seams, thrown with arm-side run and sink, a few mph slower than a four-seamer, used to induce ground balls.
Two-sided Laplace transform
The two-sided Laplace transform, also called the bilateral Laplace transform, is an integral transform integrating over the whole real line, used for non-causal signals and linear time-invariant systems.
Two-sided market
A two-sided market is one where two distinct groups, such as cardholders and merchants, interact through an intermediating platform whose price structure, not just total fees, drives participation.
Two-spirit
Two-spirit, often abbreviated 2S, is a pan-Indigenous umbrella term for diverse gender identities, sexual orientations, and spiritual roles, coined in 1990 at a gathering near Winnipeg, Manitoba.
Two-stage anaerobic digestion
Two-stage anaerobic digestion (TSAD), proposed by Pohland and Ghosh in 1971, runs acid formation and methane formation in two separately optimized reactors, typically raising methane yields by 10–30%.
Two-stage cluster sampling
Two-stage cluster sampling is a survey design in which clusters are sampled first and units subsampled within each, cutting fieldwork cost for dispersed populations while losing precision.
Two-stage sampling
Two-stage sampling is a probability sampling design that first samples larger units, such as census enumeration areas, then samples smaller units, such as households, within them.
Two-stage stochastic optimization
Two-stage stochastic optimization, also called stage stochastic optimization, is an operations research method splitting decisions into first-stage actions and second-stage recourse after uncertainty is observed.
Two-state quantum system
A two-state quantum system, also called a two-level system, is the simplest nontrivial quantum system, realized as electron spin, photon polarization, or the ammonia molecule's two configurations.
Two-state solution
Two-state solution: proposed framework for an Israeli-Palestinian peace with Israel and a Palestinian state side by side on pre-1967 lines, Jerusalem as shared capital; rooted in the 1947 partition plan and Oslo Accords, still unimplemented.