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Sigma Gamma Rho
Sigma Gamma Rho Sorority, Inc. (ΣΓΡ), a historically Black sorority of the Divine Nine, founded November 12, 1922 at Butler University, Indianapolis, by seven educators; incorporated 1929, joined NPHC 1937, slogan "Greater Service, Greater Progress."
Sigma Geminorum
Sigma Geminorum is a bright RS Canum Venaticorum binary star in Gemini, consisting of a magnetically active K1 giant and an unseen companion in a roughly 19.6-day orbit.
Sigma Healthcare
Sigma Healthcare is an Australian full-line pharmaceutical wholesaler and pharmacy franchisor founded in Melbourne in 1912; it completed a merger with Chemist Warehouse Group in February 2025.
Sigma Nu
Sigma Nu (ΣΝ) is a men's college social fraternity founded January 1, 1869 at Virginia Military Institute by Hopkins, Quarles, and Riley as the anti-hazing Legion of Honor, with over 150 chapters and 270,000 initiates.
Sigma Octantis
Sigma Octantis, officially named Polaris Australis, is a faint yellow-white giant star in Octans and the closest naked-eye star to the south celestial pole, though at magnitude 5.47 it is too dim for navigation.
Sigma Orionis
Sigma Orionis is a multiple star system of at least five components just below Orion's Belt, whose radiation illuminates the Horsehead Nebula, with a surrounding young cluster.
Sigma Orionis cluster
The Sigma Orionis cluster is a young star cluster around the multiple star Sigma Orionis, below Orion's Belt, containing about 700 stars, brown dwarfs, and planetary-mass objects.
Sigma Phi Delta
Sigma Phi Delta (ΣΦΔ) is an international professional-social fraternity for engineering students, founded on April 11, 1924, at the University of Southern California and admitting only students seeking engineering degrees.
Sigma Phi Epsilon
Sigma Phi Epsilon (SigEp), national social fraternity founded November 1, 1901 at Richmond College (now University of Richmond) by 12 men including Carter Ashton Jenkens; known for the 1991 Balanced Man Program and over 15,000 undergraduates.
Sigma Plan
The Sigma Plan (Dutch: Sigmaplan) is a flood protection and nature restoration program of the Flemish government, launched in 1977 after the January 1976 storm surge floods along the tidal Scheldt in Belgium.
Sigma Scorpii
Sigma Scorpii (σ Sco) is a quadruple star system in Scorpius, a close 33-day binary of two B-type stars plus two fainter companions, shining at magnitude +2.8.
Sigma with Armanni Reign
Sigma with Armanni Reign is the touring billing of English drum and bass duo Sigma with Philadelphia MC Armanni Reign, known for the 2014 UK number ones "Nobody to Love" and "Changing".
Sigma-Aldrich
Sigma-Aldrich, also known as MilliporeSigma, was an American research-chemical and laboratory-supplies company formed in 1975 and headquartered in St. Louis, Missouri, acquired by Merck KGaA in 2015 for about $17 billion.
Sigma-delta ADC
The sigma-delta ADC (Σ-Δ ADC, also written delta-sigma ADC) is an analog-to-digital converter that oversamples with a coarse 1-bit quantizer in a feedback loop, achieving high resolution for precision, low-bandwidth measurement.
Sigma-E factor negative regulatory protein RseA
RseA (regulator of sigma E) is the inner membrane anti-sigma factor of Escherichia coli that sequesters sigma-E (RpoE); envelope stress triggers DegS, RseP, and ClpXP proteolysis of RseA, freeing sigma-E to transcribe envelope stress genes.
Sigmar Gabriel
Sigmar Gabriel (born 1959, Goslar) is a German SPD politician: Lower Saxony minister-president 1999–2003, SPD chairman 2009–2017, Vice Chancellor 2013–2018, Foreign Minister 2017–2018, now Atlantik-Brücke chairman and supervisory board member at Deutsche Bank, Siemens Energy, and Rheinmetall.
Sigmaringen enclave
Sigmaringen enclave: the Hitler-created French exile territory at Sigmaringen castle, Germany, housing Pétain, de Brinon's governmental delegation, and about a thousand Vichy collaborators from September 1944 to April 1945.
SigmaSense
SigmaSense is an Austin, Texas-based sensing semiconductor company founded in 2015 by touchscreen veterans, known for software-defined capacitive sensing and later battery and power sensing.
Sigmatek
Sigmatek (SIGMATEK GmbH & Co KG) is an Austrian manufacturer of complete automation systems for industrial machine and plant engineering, founded in 1988 and headquartered in Lamprechtshausen near Salzburg.
Sigmatropic reaction
A sigmatropic reaction, also called a sigmatropic shift, is a pericyclic reaction in which a σ-bonded atom or group migrates across a π electron system, as in the Claisen and Cope rearrangements.
Sigmatropic rearrangement
A sigmatropic rearrangement is a pericyclic reaction in which a sigma bond flanked by pi systems migrates to a new position in a single concerted step governed by orbital symmetry.
SIGMET
SIGMET, or Significant Meteorological Information, is an ICAO-standard aviation weather warning issued by a meteorological watch office for hazards such as volcanic ash and severe turbulence.
Sigmodon
Sigmodon, the cotton rats, is a genus of sigmodontine rodents described in 1825, ranging from the southern United States to northern South America, with about 13–14 species, S-shaped molars, and wide use as laboratory models for viral disease.
Sigmoid colectomy
Sigmoid colectomy is an operation that removes the sigmoid colon to treat diverticulitis, tumors, or volvulus, with laparoscopic resection now preferred in the elective setting.
Sigmoid colon
Sigmoid colon: S-shaped terminal segment of the large intestine, 25-40 cm, joining descending colon to rectum at S3; intraperitoneal via sigmoid mesocolon, supplied by IMA sigmoidal branches, prone to diverticulitis.
Sigmoid function
The sigmoid function, also called the logistic function or squashing function, is an S-shaped curve mapping real numbers into (0, 1), used in logistic regression and neural networks.
Sigmoidoscopes
Sigmoidoscopes, rigid or flexible endoscopes with a light and camera, examine the rectum and sigmoid colon. Flexible versions, introduced by Overholt in 1962, screen for distal colorectal cancer with low complication rates.
Sigmoidoscopy
Sigmoidoscopy (flexible sigmoidoscopy) is endoscopic examination of the rectum and lower colon with a flexible camera tube. Used for symptoms and CRC screening; RCTs show 28% mortality reduction, benefit limited to distal colon; perforation about 1 in 50,000.
Sigmund Fraenkel
Sigmund Fraenkel, also written Siegmund Fränkel, was an Austrian physician, chemist, and pharmacologist who taught medical chemistry at Vienna and wrote Die Arzneimittel-Synthese, first published in 1901.
Sigmund Freud
Sigmund Freud (1856–1939), Austrian neurologist and founder of psychoanalysis; born in Freiberg, Moravia, worked in Vienna, fled the Nazis in 1938, died in London; wrote The Interpretation of Dreams and Studies on Hysteria.