What is hold fast?

 
In algal family phaeophycae, the basic body of the plant is represented by a holdfast, stipe and lamina. holdfast is the stalk of the plant . (OR) 
An organ or structure of attachment, especially the basal, rootlike formation by which certain seaweeds or other algae are attached to a substrate.(or)
A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate. Holdfasts vary in shape and form depending on both the species and the substrate type.