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1. The uppercase of the Greek letter sigma. 2. In statistics, the symbol for summation (2). SEE: σ.


1. The lowercase of the Greek letter sigma. 2. In statistics, the symbol for standard deviation. SEE: Σ.


[L. signa, mark] 1. Symbol for the element sulfur. 2. In prescription writing, the symbol indicating the instructions to the patient that the pharmacist will place on the dispensed medicine. 3. Smooth, in reference to bacterial colonies. 4. Spherical or spherical lens. 5. Subject (pl. Ss); a participant in an experiment. 6. Symbol for siemens.


semis, (L. half); sinister, (L. left); soluble.

S1, S2

Normal first and second heart sounds.


A ventricular gallop heard after the second heart sound (S2). It is a sign of heart failure.


The fourth heart sound, a presystolic gallop (also known as atrial gallop), which precedes the first heart sound (S1). It is abnormal and usually indicative of a stiff (noncompliant) ventricle.

S-A, SA, S.A.



slow-acting antirheumatic drug.


short-acting beta agonist.

Sabiá virus

(să-bē-ah′) [Sabiá, a village outside São Paulo, Brazil] SEE: under virus.

Sabin vaccine

(sā′bĭn) [Albert Bruce Sabin, Russian-born U.S. virologist, 1906–1993] SEE: Live oral poliovirus vaccine.

Sabouraud dextrose agar

(să-boo-rō′) An acidic agar with a high dextrose content. It is used in microbiology to cultivate fungi and yeasts.


(sab′yŭ-lŭs) [L. sabulosus, sandy] Gritty; sandy.


(sak) [L. saccus, sack, bag] A baglike part of an organ, a cavity or pouch, sometimes containing fluid. SYN: saccus. SEE: cyst.

air s. In a mammalian lung, an informal term for pulmonary alveolus. SEE: under alveolus.

allantoic s. The expanded end of the allantois, well developed in birds and reptiles.

alveolar s. Pulmonary alveolus.

amniotic s. The fetal membrane that encloses the developing fetus and contains amniotic fluid. SEE: chorion.

chorionic s. The outer fetal membrane that encloses the developing embryo.

conjunctival s. The cavity, lined with conjunctiva, that lies between the eyelids and the anterior surface of the eye.

dental s. The mesenchymal tissue surrounding a developing tooth.


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