The Collaborative International Dictionary of English v.0.48:

Helicoid \Hel"i*coid\, a. [Gr. ?; ?, ?, spiral + ? shape: cf. F.
   h['e]lico["i]de. See Helix.]
   [1913 Webster]
   1. Spiral; curved, like the spire of a univalve shell.
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   2. (Zool.) Shaped like a snail shell; pertaining to the
      Helicid[ae], or Snail family.
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   Helicoid parabola (Math.), the parabolic spiral.
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The Collaborative International Dictionary of English v.0.48:

Helicoid \Hel"i*coid\, n. (Geom.)
   A warped surface which may be generated by a straight line
   moving in such a manner that every point of the line shall
   have a uniform motion in the direction of another fixed
   straight line, and at the same time a uniform angular motion
   about it.
   [1913 Webster]