US terms: slip stitch (sl. st.)
UK terms: slip stitch (ss)
Usually marked by a dot or a short dash.
As it’s also called a closing stitch, there is frequent confusion as they are two different stitches and so is a question of terminology. Usually a closing stitch is the term given to slip stitches which close something – and so they connect two pieces, a circle, or are on the last row which finishes off a crochet project. It even happens that a closing slip stitch is called a single crochet, especially if this a translation from a foreign language, where there are great differences in the labels used.
We insert the hook in the crochet piece, and pull the yarn through both stitches on the hook – the one into which the hook has been inserted and that already on it. Now we have a slip stitch.
This requires one move, which makes the slip stitch different from the single crochet stitch.