| threshold substance | Any material (e.g., glucose) that is excreted in the urine only when its plasma concentration exceeds a certain value, termed its threshold. Synonym: threshold body. (05 Mar 2000) |
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| threshold trait | A trait that falls into natural groups that originate not in categorically distinct causes but in whether or not the outcome attains critical values; e.g., gallstones may result from a categorical cause or from unusual levels of causal factors that themselves show no evidence of grouping. Synonym: liminal trait. (05 Mar 2000) |
| transmission threshold | <epidemiology> Occurs for a parasite when the basic reproductive rate Ro is equal to 1. Below this threshold level the disease is unable to maintain itself within the host population. Typically, for directly transmitted parasites there is a transmission threshold for the host population size. (05 Dec 1998) |
| erythema threshold | The amount of radiation that causes redness of the skin. (09 Oct 1997) |
| fibrillation threshold | Least intensity of an electrical stimulus that will initiate fibrillation. (05 Mar 2000) |
| fusion-inferred threshold test | Employment of the phenomenon of cerebral fusion of binaural sounds to substitute for conventional masking in hearing testing. Synonym: FIT test. (05 Mar 2000) |
| light differential threshold | The smallest difference in light intensity that can be appreciated. (05 Mar 2000) |
| atomic number | <chemistry> Symbol Z. The number of protons in the nucleus of an atom and also its characteristic atomic number. The atomic numbers of the known elements form a complete series from 1 (hydrogen) to 103 (lawrencium). (16 Dec 1997) |
| Avogadro's number | <physics> The number of molecules in a mole (gram molecular weight) of a substance, equals 6.02 x 1023 molecules. (09 Oct 1997) |
| Brinell hardness number | A number related to the size of the permanent impression made by a ball indenter of specified size (usually 10 mm in diameter) pressed into the surface of the material under a specified load: where P = applied load in kg, D = diameter of the ball in mm, and d = diameter of the impression in mm. (05 Mar 2000) |
| burst number | The number of viral particles that emerge from a cell after a viralinfection has burst it open. (09 Oct 1997) |
| Mach number | A number representing the ratio between the speed of an object moving through a fluid medium, such as air, and the speed of sound in the same medium. (05 Mar 2000) |
| magnetic mach number | <physics> A dimensionless number equal to the ratio of the velocity of a fluid to the velocity of Alfven waves in that fluid. (13 Nov 1997) |
| mass number | The mass of the atom of a particular isotope relative to hydrogen-1 (or to 1/12 the mass of carbon-12), generally very close to the whole number represented by the sum of the protons and neutrons in the atomic nucleus of the isotope (indicated in the name or symbol of the isotope; e.g., oxygen-16, 16O); not to be confused with the atomic weight of an element, which may include a number of isotopes in natural proportion. (05 Mar 2000) |
| Reichert-Meissl number | An index of the volatile acid content of a fat; the number of milliliters of 0.1 n KOH required to neutralise the soluble volatile fatty acids in 5 g of fat that has been saponified, acidified to liberate the fatty acids, and then steam-distilled. Synonym: volatile fatty acid number. (05 Mar 2000) |