Colloquially usually known by the trade-name Ty-wrap (often written as tie-wrap), I understand this to be a cable tie — a plastic band that is tightened round a cable or several to form a neat bundle, or to attach to other objects. These are available in various sizes, from quite tiny to pretty huge.
In my UK experience, they are often (but not exclusively) made of nylon, but here in France they appear to have been made from Rilsan plastic — or at least, that name seems to have stuck for them!