This is what I found when I googled ply:

The word ply needs to be explained. If the wool is 2 ply, then it means the yarn consists of 2 plies. A ply pertains to a single strand of yarn. ...

"My understanding of ply means that each ply a "single" cannot be broken down into more strands, if you try pulling one of the singlse apart, you get fiber. By two ply I was refering to two singles spun together making for a more dense or heavier fabric appearance. "