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 ]