The Collaborative International Dictionary of English v.0.48:

Holdfast \Hold"fast`\, n.
   1. Something used to secure and hold in place something else,
      as a long flat-headed nail, a catch a hook, a clinch, a
      clamp, etc.; hence, a support. "His holdfast was gone."
      --Bp. Montagu.
      [1913 Webster]

   2. (Bot.) A conical or branching body, by which a seaweed is
      attached to its support, and differing from a root in that
      it is not specially absorbent of moisture.
      [1913 Webster]

WordNet (r) 3.0 (2006):

holdfast
    n 1: restraint that attaches to something or holds something in
         place [syn: fastener, fastening, holdfast, fixing]