A yarn in which two or more single yarns are twisted together (fold) in one operation, e.g., two-fold yarn, three-fold yarn, etc
Note: In some sections of the textile industry, e.g., the marketing of hand-knitting yams, these yarns are referred to as two-ply, three-ply, etc.
also termed doubled yarn; plied yarn
Textile Resource (http://www.textile.org.uk)