About the Rigid Heddle Loom
Anything that holds warp threads under tension 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 a rigid heddle loom and other types of looms.
Although he says that a rigid heddle 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. Weave without the rigid heddle at all. Combined with pick up sticks and string heddles, the weaving possibilities are endless. Inkle weaving, tablet weaving, complicated weave structures, tapestry and intricate pick up designs can all be done just by using the frame.
Side note: Although shaft and harness are often used interchangeably, the proper term is shaft. A harness is a frame that holds groups of shafts (as in a drawloom).
The "holes" in the rigid heddle are actually the heddles. The "slots" are the spaces between the heddles. It's called a rigid heddle because, unlike shaft looms, the heddles do not move from left to right.