In order to understand what tapestry yarn may mean, it is first necessary to unravel the meaning of the word tapestry. Tapestry means several different things. First, it is the name of a specific style of weaving, and you will often find tapestry weave or kilim weave contrasted to plain weave.
Tapestry weave is weft-faced, with the warps invisible, and has characteristic joins for color changes called diagonal, dovetailed, interlocked, and slit. In this usage, tapestry can be applied to medieval European wall hangings, Navajo blankets, and Middle Eastern carpets, among other fabrics in which discontinuous weft is used to form scenes or patterns. There is a particular style of loom called a tapestry loom on which this work is done.
So, going by this first usage, tapestry yarn can be applied to any yarn used on tapestry looms to make tapestries. Traditionally, this has included wool, silk, linen, and cotton fiber. Tapestry yarn is purchased specifically according to whether it is intended to serve as warp or weft, because different qualities are needed for each.
Recently, tapestry has come to be applied to fabric constructed on a backing and used for wall hangings or upholstery; that is, needlepoint and crewel embroidery that usually depicts scenes or pictures. In the this latter day usage, tapestry yarn is the name of a specific type of yarn, alternatively called tapestry wool, a 3- or 4-ply wool yarn often chosen by crafters for crewel embroidery and making needlepoint. Tapestry yarn is sold in skeins of 8.8 or 11 yards (~8 or 10 m) and 40, 42.7, or 62 yard (~37, 39, or 56 m) hanks, depending on the manufacturer. Tapestry yarn is available in a wide array of colors.
In addition, tapestry refers to a specific weight of fiber, called tapestry braid in the following scheme used by Kreinik, a thread manufacturer:
Kreinik's metallic braid, which comes in a variety of styles, including confetti and glow-in-the-dark, is recommended for use as a tapestry yarn. In addition, their tapestry braid is suggested for use in cross-stitch, crochet, as companion yarn in knitting, and for collage and other crafts. Shorter lengths are recommended for doing needlework with this type of tapestry yarn in order to limit the wear on them caused by repeated pulling through the base fabric or canvas.