Stitch (textile arts)

In the textile arts, a stitch is a single turn or loop of thread, or yarn. Stitches are the fundamental elements of sewing, knitting, embroidery, crochet, and needle lace-making, whether by hand or machine. A variety of stitches, each with one or more names, are used for specific purposes.
Sewing, embroidery, and lace.
All stitches:
These stitches and their variations are named according to the position of the needle and direction of sewing ("running stitch", "backstitch"), the form or shape of the stitch ("chain stitch", "feather stitch") or the purpose of the stitch (tailor's tack, "hem stitch").
Sewing machine stitches are classified by their structure: 
Fancy machine stitches mimic traditional hand stitches using variations on the basic stitches.
Knitting.
In knitting, a stitch is a single loop of yarn, secured to the loops beside it to form a row or "course" of stitches and to the loops above and below it to form a "wale".. 
In securing the previous stitch in a wale, the next stitch can pass through the previous loop either from below or above. If the former, the stitch is denoted as a knit stitch or a plain stitch; if the latter, as a purl stitch. The two stitches are related in that a knit stitch seen from one side of the fabric appears as a purl stitch on the other side.
Crochet.
In crochet, stitches are made by pulling a loop of thread through the work with a crochet hook. Crochet stitches are named based on their structure. In the English-speaking crochet world, basic stitches have different names that vary by country. The differences are usually referred to as UK/US or British/American.