| horizon | 1. The circle which bounds that part of the earth's surface visible to a spectator from a given point; the apparent junction of the earth and sky. "And when the morning sun shall raise his car Above the border of this horizon." (Shak) "All the horizon round Invested with bright rays." (Milton) 2. <astronomy> The unbroken line separating sky and water, as seen by an eye at a given elevation, no land being visible. 3. <geology> The epoch or time during which a deposit was made. "The strata all over the earth, which were formed at the same time, are said to belong to the same geological horizon." (Le Conte) 4. The chief horizontal line in a picture of any sort, which determines in the picture the height of the eye of the spectator; in an extended landscape, the representation of the natural horizon corresponds with this line. Apparent horizon. See Apparent. Artificial horizon, a level mirror, as the surface of mercury in a shallow vessel, or a plane reflector adjusted to the true level artificially; used chiefly with the sextant for observing the double altitude of a celestial body. Celestial horizon. <astronomy> See def. 2, above. Visible horizon. See definitions 1 and 2, above. Origin: F, fr. L. Horizon, fr. Gr. (sc) the bounding line, horizon, fr. To bound, fr. Boundary, limit. Source: Websters Dictionary (01 Mar 1998) |
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| horizontal | 1. Pertaining to, or near, the horizon. "Horizontal misty air." 2. Parallel to the horizon; on a level; as, a horizontalline or surface. 3. Measured or contained in a plane of the horizon; as, horizontal distance. Horizontal drill, a drilling machine having a horizontal drill spindle. Horizontal engine, one the piston of which works horizontally. Horizontal fire, the fire of ordnance and small arms at point-blank range or at low angles of elevation. <physics> Horizontal force, the distance in a horizontal plane to which a gun will throw a projectile. Horizontal water wheel, a water wheel in which the axis is vertical, the buckets or floats revolving in a horizontal plane, as in most turbines. Origin: Cf. F. Horizontal. Source: Websters Dictionary (01 Mar 1998) |
| horizontal atrophy | A progressive loss of alveolar and supporting bone surrounding the teeth, beginning at the most coronal level of the bone. Synonym: horizontal resorption. (05 Mar 2000) |
| horizontal beam film | A radiograph made with the central axis of the X-ray beam parallel to the floor, able to show an air-fluid level. (05 Mar 2000) |
| horizontal cell | A type of nonspiking interneuron found in the retina, named for its morphology. Horizontal cells process the information from a large number of photoreceptors and synapse onto ganglion cells and other cell types in the retina. (18 Nov 1997) |
| horizontal cell of Cajal | A small fusiform cell found in the superficial layer of the cerebral cortex with its long axis placed horizontally. Synonym: Cajal's cell. (05 Mar 2000) |
| horizontal cells of retina | Cell's in the outer part of the inner nuclear layer of the retina that lie with their axes more or less parallel with the surface. They are thought to connect the rods of one part of the retina with cones of another part. (05 Mar 2000) |
| horizontal classification | <zoology> Classification which stresses grouping together taxa in a similar stage of evolution, rather than location on the same phyletic line. See: Vertical classification. (09 Jan 1998) |
| horizontal fissure of cerebellum | Horizontal fissure that divides the ansiform lobule into its major parts, crus I (superior semilunar lobule) and crus II (inferior semilunar lobule). Synonym: fissura horizontalis cerebelli, great horizontal fissure. (05 Mar 2000) |
| horizontal fissure of right lung | The deep fissure that separates the upper and middle lobes of the right lung. Synonym: fissura horizontalis pulmonis dextri, horizontal fissure of right lung, minor fissure. (05 Mar 2000) |
| horizontal fracture | A fracture of the facial bones in which there is a horizontal fracture at the base of the maxillae above the apices of the teeth. Synonym: horizontal fracture, Le Fort I fracture. (05 Mar 2000) |
| horizontal growth phase | An early stage of development of cutaneous melanoma by intraepidermal spread of atypical melanocytes. (05 Mar 2000) |
| horizontal osteotomy | An osteotomy performed intraorally for genioplasty; the inferior aspect of the anterior mandible is advanced or retruded by movement of the free segment. (05 Mar 2000) |
| horizontal overlap | The projection of the upper anterior and/or posterior teeth beyond their antagonists in a horizontal direction. Synonym: overjet, overjut. (05 Mar 2000) |
| horizontal part of duodenum | pars horizontalis duodeni, inferior part of duodenum, pars inferior duodeni |
| sakuc horizon | <chemistry> A layer 6 inches or more thick comprised of secondary soluble salts. (17 Dec 1997) |
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| soil horizon | A distinct layer of soil, more or less parallel with the soil surface, having similar properties such as colour, texture and permeability, the soil profile is subdivided into the following major horizons: A-horizon, characterised by an accumulation of organic material. B-horizon, characterised by relative accumulation of clay iron, organic matter or aluminum. C-horizon, the undisturbed and unaltered parent material. (Some soils have an E-horizon, characterised by leaching of organic and other material.) (09 Oct 1997) |
| spodic horizon | A subsurface layer of soil characterised by the accumulation of aluminum oxides (with or without iron oxides) and organic matter, a diagnostic horizon for spodosols. (09 Oct 1997) |
| Streeter's developmental horizon | A term borrowed from geology and archeology by Streeter to define 23 developmental stages in young human embryos, from fertilization through the first 2 months; each horizon spanned 2 to 3 days and emphasized specific anatomic characteristics, to avoid discrepancies in the determination of age and body dimensions. Origin: G.L. Streeter (05 Mar 2000) |
| type horizon | <zoology> The geological stratum from which the name-bearing type of a nominal species or subspecies was collected. (09 Jan 1998) |