packthread(n = noun.artifact) - a strong three-ply twine used to sew or tie packages;
is a kind of string, twine

   A strong three-ply thread or small twine used to sew or tie packs or packages. [WordNet 1.5]

packthread, n. stout thread for sewing or tying up packs.

N  filament, line, fiber, fibril, funicle, vein, hair, capillament, cilium, cilia, pilus, pili, tendril, gossamer, hair stroke, veinlet, venula, venule, wire, string, thread, packthread, cotton, sewing silk, twine, twist, whipcord, tape, ribbon, cord, rope, yarn, hemp, oakum, jute, strip, shred, slip, spill, list, band, fillet, fascia, ribbon, riband, roll, lath, splinter, shiver, shaving, beard, ramification, strand, filamentous, filamentiferous, filaceous, filiform, fibrous, fibrillous, thread-like, wiry, stringy, ropy, capillary, capilliform, funicular, wire-drawn, anguilliform, flagelliform, hairy, taeniate, taeniform, taenioid, venose, venous.