False negative (health) and V(1) (iou)

False negative (health)

A result that appears negative but fails to reveal a situation. An example of a false negative: a particular test designed to detect cancer of the toenail is negative but the person has toenail cancer.

V(1) (iou)

V, v. [vi:]
The twenty-second letter of the modern English alphabet and the twentieth of the ancient Roman one, of which U is a differentiated form. Orig. an adoption of the early Greek vowel-symbol V, now also represented by U and Y, it also represented in Latin a semi-vocalic sound [w] (corresp. to the Greek digamma) which gradually changed to a bilabial [w] and later a labiodental [v] consonant. English use of V first became established in Middle English under French influence, but without a formal distinction of use from U, with which it continued to share the functions of representing both a vowel and a consonant until about 1700 (see U, U) before being regularly distinguished as the consonant. The sound now normally represented by the letter is a voiced labiodental fricative consonant. Pl. V's, Vs. See also VEE.
I.
The letter and its sound.
The shape of the letter.
V aerial, V antenna an aerial in which the conductors form a large horizontal V that transmits principally along its axis. V antenna: see V aerial above. V belt a belt which is V-shaped in cross-section in order to give better traction on a pulley. V-block a metal block with a V-shaped recess cut in it to hold a cylindrical object while it is being worked on. V-eight, V-8 a V-engine with eight cylinders; a motor vehicle with such an engine. V-engine an internal-combustion engine with the cylinders arranged in two rows at an angle to each Other, forming a V-shaped cross-section. V-moth a geometrid moth, Semiothisa wauaria, which has a dark V-shaped mark on the pale forewing. V-neck (a pullover or Other garment with) a neckline in the shape of a V. V-pug a small geometrid moth, Chloroclystis v-ata, which has a V-shaped mark on the forewing. V-shaped adjective having a shape or a cross-section like the letter V. V-six, V-6 a V-engine with six cylinders; a motor vehicle with such an engine. V-thread a screw thread which is V-shaped in profile.
II. Symbolical uses.
Used to denote serial order; applied e.g. to the twenty-second (or often the twenty-first, either I or J being omitted) group or section, sheet of a book, etc.
b. V-agent, any of a group of organophosphorus nerve gases having anticholinesterase activity; VX, a liquid V-agent produced esp. by the US in the 1960s.
The roman numeral for five.
Particle Physics. (Cap. V.) Any of a group of heavy unstable particles, now identified as hyperons and kaons, that produce characteristic V-shaped tracks when they decay. obsolete exc. Hist.
III.
Abbrevs.: V = (US) a five-dollar note; (Chemistry) vanadium; [German] Vergeltungswaffe reprisal weapon (esp. in V-1, a type of German flying bomb used in the war of 1939-45, V-2, a type of German rocket-powered missile used in the war of 1939-45); (Ecclesiastical) versicle; victory; volt. v. = verb; verse; verso; versus; very; vide. v = velocity; volume.

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