Monotype is a technique, while monoprint is a concept.
Let me explain.
A monotype is a technique that uses a smooth plate made of plastic or metal to transfer ink to paper. The plate has no texture, no incised lines, no etching. You don't even need a press to transfer ink to paper. Some do not even consider it to be a true printmaking process since you cannot duplicate a monotype.
Each impression from the smooth plate will be unique.
|Two Monotype plates ready for printing; the light|
image on the left will be printed 1st as an under-painting.
A monoprint is one of a series of prints. There is a commonality to each impression, yet, each is unique but more like a variation on a theme rather than totally new image. That commonality is the underlying image from an etched plate, or other printmaking plate or stone.