Anything that holds warp threads tightly is a loom. The difference between looms is how the warp is held and how the sheds are created.
A rigid heddle loom is, in effect, a frame loom. Generally, it has 4 sides but not always. You can weave anything on a frame loom.
Here's a nice video of Tim from New Voyager Trading telling us the difference between an RH loom and other types of looms.
Although he says that an RH loom is limiting, this isn't necessarily true. You can weave anything on a frame loom. You can use more than one rigid heddle as well. You can weave 4 shaft drafts using 3 Rigid Heddles! Combined with pick up sticks and string heddles, the weaving possibilities are endless.