| fluoride number | The percent inhibition of pseudocholinesterase produced by fluorides; used to differentiate normal from atypical pseudocholinesterases. See: dibucaine number. (05 Mar 2000) |
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| f-number | <radiobiology> In optics, denotes the ratio of the equivalent focal length of an objective lens to the diameter of its entrance pupil. (09 Oct 1997) |
| linking number | A property of a long biopolymer (such as duplex DNA) equal to the number of twists (related to the frequency of turns around the central axis of the helix) plus the writhing number. (05 Mar 2000) |
| Loschmidt's number | The number of molecules in 1 cm3 of ideal gas at 0캜 and 1 atmosphere of pressure; Avogadro's number divided by 22,414 (i.e., 2.6868 × 1019 cm-3). (05 Mar 2000) |