| covalent | Denoting an interatomic bond characterised by the sharing of 2, 4, or 6 electrons. (05 Mar 2000) |
|---|---|
| covalent bond | A bond between two or more atoms that is provided by electrons that travel between the atoms' nuclei, holding them together but keeping them a stable distance apart. (09 Oct 1997) |
| covalently closed circular DNA | <molecular biology> A circular molecule of double-stranded DNA which is supercoiled, or coiled up on itself due to internal tensions, because there are no breaks in the phosphate backbone (upon which the nucleotide bases are mounted) to relieve the tensions and allow it to form an open circle. (09 Oct 1997) |
| cove plane | A classic description of terminal inversion of the electrocardiographic T wave with the initial portion arched above the baseline and the terminal portion below it, the former being rounded and the latter pointed. (05 Mar 2000) |
| cover | Anything that provides protection from predators or ameliorates adverse conditions of streamflow and/or seasonal changes in metabolic costs. May be instream cover, turbulence, and/or overhead cover, and may be for the purpose of escape, feeding, hiding, or resting. (09 Oct 1997) |
| cover class | A category into which plant species would fit based upon their percent areal cover, the cover classes used (midpoints in parentheses) are T = <1% cover (0), 1 = 1-5% (3.0), 2 = 6-15% (10.5), 7 = 51-75% (63.0), 6 = 76-95% (85.5), 7 = 96-100% (98.0). (09 Oct 1997) |
| cover glass | A thin glass disk or plate covering an object examined under the microscope. Synonym: coverslip. (05 Mar 2000) |
| cover test | A test used for objective demonstration of ocular deviation in strabismus; may be performed by two methods: the cover-uncover test and the alternate cover test. (05 Mar 2000) |
| cover-uncover test | A test to detect strabismus; the patient's attention is directed to a small fixation object, one eye is covered and after a few seconds, uncovered; if the uncovered eye moves to see the picture, strabismus is present. (05 Mar 2000) |
| coverage | A measure of the extent to which the services rendered cover the potential need for these services in a community; applied specifically to such services as immunization in developing countries. (05 Mar 2000) |
| covering | A surrounding layer; something that covers or encloses, forming an outer layer. See: tunica. Coverings of spermatic cord, coverings of the spermatic cord, including external and internal spermatic fasciae, and cremasteric muscle and fascia. Synonym: tunicae funiculi spermatici. (05 Mar 2000) |
| coverslip | <microscopy> Thin glass, plastic, sapphire, SiO, NaCl, etc. Shaped into circles, squares, or rectangles for covering the specimen. Glass coverslips, grade no. 1 1/2, contain a high proportion of covers from 0.17 to 0.18 mm in thickness, close to the lens designer's specifications for employment of achromatic and apochromatic objectives of high numerical aperture. The thickness and refractive index of the glass coverslip are important because certain of their values have been assumed by the designer and manufacturer. These values have been specified by ASTM: thickness, commercial number 1 1/2: 0.16 to 0.19 mm, refractive index measured at the sodium D lines (589.3 nm), 1.523(0.005). (05 Aug 1998) |
| covert sensitization | Aversive conditioning or training to rid onself of an unwanted behaviour during which the patient is taught to imagine unpleasant and related aversive consequences while engaging in the unwanted habit. (05 Mar 2000) |