A (usually hand-powered) device for knitting a lot faster than you can with 2 knitting needles. It consists of a long row of hooked needles and a kind of sliding shuttle thing that does the knitting.
/----\
| | -->
------
JJJJ^^^^^^JJJJJJJJJJJJJJJ---,
######################## \
########################
########################
#####/
The yarn is laid along the row of hook needles and the shuttle slides along it, with a kind of brush/gear mechanism that knits the row. Then the yarn is laid going the other way.
It is surprisingly fast and flexible. My mom knits sweaters using one, arms and all, and multicolored patterns. It's possible to remove the knitting from the machine and transfer it to a pair of needles so the seams can be done properly, not just sewn up.
They come in various hook densities, so some machines are better at doing fine knitting and others are better at doing coarse knitting or heavy yarn. I assume they also have motorized, fully-automatic knitting machines in textile factories.