Aran weight yarn is a regional term describing a yarn that knits at 18 stitches over 4" (10 cm) in stockinette stitch. Not everybody describes this weight of yarn as "Aran" weight; some people call it "worsted" or "heavy worsted." If you compare the terminology used by manufacturers and retailers to describe weights of yarn, you will find that there is some variation: to some, "worsted" is anything that knits from 16 to 20 stitches per 4" (10 cm). To others, "worsted" means 19 to 20 stitches. Some people will distinguish between "bulky" and "chunky" yarns, and others won't.
In general, when you pick yarn for a project and gauge is important, don't rely on the manufacturer's or retailer's descriptive terms alone. You should confirm--by swatching, where possible--that the yarn you intend to use will knit at the pattern's required gauge.
