You see this word monoplaner bandied about a lot on eBay etc, but does it actually mean anything in cycling terms!? I can't find it's usage in any other engineering or technical reference. Made up Campagnolo word!?
I think it refers to the fact that both sides of the caliper rotate around the same point along the pivot pin.
The forked pivot mount of the right hand side straddles the single pivot mount on the left, hence a singular plane of actuation - mono planar.