Sewing: One of the controls on a sewing machine. Surprisingly enough, it controls the length of the stitching. More specifically, it controls the amount of fabric the feed dogs push through before the needle comes down again.
In a straight stitch, the length of the stitches you use depends on the fabric you're sewing. A closely woven fabric will require a shorter stitch length than a coarser one. This rule does not apply when doing specialised items such as buttonholes.