A 'loom' is a device upon which weaving is performed. This can be a very low technology solution involving only a few sticks, or may involve many dodits and whatsits that make the process of weaving semi-automated or enable greater variation in weaving types produced.
The term loom is used for a variety of forms of weaving and related activities - cloth is woven on looms, but narrowwares - narrow decorative bands - can be woven on specialist looms, and devices upon which sprang is created might also be called a loom, although 'frame' is more often used.
warp weighted loom
three shaft loom
Tabletweaving can be performed strung between any two immobile objects, leading to one very simple method of attachment called the "backstrap loom" where one end of the weaving is tied to a immobile object (eg doorknob) and the other to a belt around the weaver's waist. This method was used traditionally in Scandanavia (ie in the 19th Century, but evidence of it's use in period is scarce. One illustration from the Manesse Codex shows a woman weaving, the far end attached to a rod on the wall, and the near end is being woven from her lover's hair. The warp both passes through a freestanding rigid heddle device and has cards strung on it, which seems superfluous as either device would be suficient for weaving narrowwares. The artistic symbolic nature of weaving her lover's hair probably outweighs accuracy in this instance.
The minimum requirements for a permanent loom (one that allows the weaver to leave the weaving without arduous setting up upon return) is two posts driven into the ground. There has been some speculation about this occuring in anglo-saxon contexts.
A standard tabletweaving loom for the medieval period was a 'band loom', a freestanding frame, which supplies the two posts but with a framework enabling them to stand on a stone floor, rather than have to be driven into the ground. The only extant band loom we have fom the medieval period is the 'osenberg loom', found in a XXth Century Scandanavian ship burial, along wit ha set of strung tabletweaving cards. Such looms are generally 3m long and 1 to 1.5 m tall, and several dozen manuscript depictions of such looms are known, some clearly showing tabletweavign cards, and others showing no cards, nor heddles, may have been used for other narrowwares techniques, or may simply represent lack of artist knowledge. Weaving is one of the standard poses of the virgin mary in medieval psalters.
Rarer in manuscipt depictions are unusual looms - one which might be warping boards, or might be a period inkle loom. Or a shorter (1m long) band loom with ratchets to enable moving the warp along with more ease.