Slipstitch (abbreviation: sl.st)
A slipstitch looks a lot like a single crochet. The difference is that you immediately pull the yarn you pick up through both loops.
In image 2 the thread does not go through 1 loop but through the picked up loop and the loop that is already on your hook.
The slip stitch can also be used to complete a magic circle or chain stitch. (See picture 3).