Dropped Stitch
A dropped stitch is a stitch that has fallen off the needle unintentionally and begun to unravel down the fabric, and should be fixed with a crochet hook.
Explanation
A dropped stitch occurs when a stitch falls off the needle without being knitted, and is often pulled downward by the yarn's own tension, forming a ladder of loose strands running down through the fabric. The longer you knit before noticing the mistake, the further the stitch runs, and the harder it becomes to fix. The usual solution is to use a crochet hook to catch the stitch and hook it back up, strand by strand, to the correct height on the needle, often using the pattern's stitch pattern as a guide.