The definition of a stitch, from The Science of Knitting (Wiley, 1914) by Ernest Tompkins:
A stitch is really the combination of loops from adjoining threads forming a fixed part of the fabric, and the duplication of which forms the whole fabric.
But a stitch is frequently considered to be the length of yarn from any point to an adjoining corresponding point, e.g. from the middle of a sinker loop [running thread, in hand-knitting] to the middle of the next sinker loop.