Skein

A length of yarn wound into a package for sale. A skein is a measured quantity with a stated weight and length, so a 100 gram skein holding 200 yards is a different amount of yarn from a 100 gram skein holding 400 yards.

This is why an estimate in yards is more useful than an estimate in skeins: yarn is sold in whole skeins, but the yardage is what the project actually consumes.