Spi·ral a.
 1. Winding or circling round a center or pole and gradually receding from it; as, the spiral curve of a watch spring.
 2. Winding round a cylinder or imaginary axis, and at the same time rising or advancing forward; winding like the thread of a screw; helical.
 3. Geom. Of or pertaining to a spiral; like a spiral.
 Spiral gear, or Spiral wheel Mach., a gear resembling in general a spur gear, but having its teeth cut at an angle with its axis, or so that they form small portions of screws or spirals.
 Spiral gearing, a kind of gearing sometimes used in light machinery, in which spiral gears, instead of bevel gears, are used to transmit motion between shafts that are not parallel.
 Spiral operculum, an operculum whih has spiral lines of growth.
 Spiral shell, any shell in which the whorls form a spiral or helix.
 Spiral spring. See the Note under Spring, n., 4.