All articles starting with P
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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.
Perimetry
Perimetry is the systematic measurement of the visual field, presenting light stimuli at many locations to detect and monitor glaucoma and localize retinal and neuro-ophthalmic disease.
Perinatal arterial stroke
Perinatal arterial ischemic stroke (PAIS, neonatal arterial ischemic stroke) is arterial-brain infarction between late gestation and day 28 of life, presenting with focal seizures, diagnosed by MRI, causing hemiparetic cerebral palsy.
Perinatal asphyxia
Perinatal asphyxia (birth asphyxia) is hypoxic-ischemic organ injury near delivery, causing about 900,000 neonatal deaths yearly; its brain form is HIE, treated with 72-hour therapeutic hypothermia at 33.5–34.5 °C.
Perinatal Care
Perinatal care: organized health care for women and newborns across pregnancy, birth, and the first 42 days postpartum. Covers midwife-led continuity models, WHO guidance, and 2023 global mortality data.
Perinatal stroke
Perinatal stroke: focal vascular brain injury from 20 weeks' gestation to 28 days postnatal, mostly arterial ischemic, presenting with neonatal seizures, diagnosed by MRI, causing hemiparetic cerebral palsy.
Perindopril
Perindopril (Coversyl, Aceon) is a once-daily ACE inhibitor approved in 1988 for hypertension, heart failure, and stable coronary artery disease; active metabolite perindoprilat, half-life 30-120 hours, common side effect dry cough.
Perineal massage
Perineal massage is a technique in which the perineum is stretched with lubricated fingers from about 34 weeks of pregnancy, modestly reducing perineal trauma and episiotomy during vaginal birth.
Perineal pain
Perineal pain is pain in the perineum between the genitalia and anus (ICD-11 GA34.01), acute after childbirth trauma or chronic via pudendal neuralgia; diagnosed clinically, treated with paracetamol, cooling, and nerve blocks.
Perineal tear
A perineal laceration during delivery is a tear of the perineum during childbirth, graded from first to fourth degree, affecting up to 9 in 10 first-time mothers.
Perineum
The perineum, or anogenital area, is the body region between the thighs containing the external genitalia and anal opening, a region important in obstetrics and surgery.
Perineum sunning
Perineum sunning (butthole sunning) is a wellness practice of exposing the perineum to sunlight, viralized by an October 2019 Instagram post claiming 30 seconds equals a day of sun exposure; no evidence supports its benefits and dermatologists warn of sunburn and aggressive skin cancer risk.
Perineural injection
Perineural injection is the deposit of medication, usually local anesthetic with or without corticosteroid, around a nerve to block conduction, relieve pain, or treat nerve disease.
Perineurial cell
A perineurial cell is a flattened cell that forms the concentric layers of the perineurium, the sheath around each peripheral nerve fascicle, creating the blood-nerve barrier.
Peringia ulvae
Peringia ulvae (laver spire shell, formerly Hydrobia ulvae) is a 4-6 mm hydrobiid mud snail of northeast Atlantic estuaries and mudflats, abundant to 300,000/m², a deposit feeder and key shorebird prey.
Perinucleolar compartment
Perinucleolar compartment (PNC): a membraneless cancer-cell nuclear body at the nucleolar periphery, 0.25–4 µm, enriched in pol III noncoding RNAs and RNA-binding proteins like PTB, correlating with metastasis.
Period (album)
Period (album) covers two albums with this title: City Girls' 2018 debut mixtape Period on Quality Control (16 tracks) and Kesha's 2025 independent sixth album . (Period) on Kesha Records, plus other minor uses.
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-instrument orchestra
A period-instrument orchestra performs orchestral repertoire on instruments of the composer's era or replicas, using historically informed practice; leading ensembles include the Academy of Ancient Music and Boston Baroque.
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 acid–Schiff stain
Periodic acid–Schiff (PAS) stain, also called PAS reaction, a histochemical method introduced by McManus in 1946 that oxidizes tissue carbohydrates to aldehydes and stains glycogen, neutral mucins, basement membranes, and fungi pink-magenta with Schiff's reagent.
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.