It *is* a great question, and one that comes up frequently. If there is nothing on the plate used to make the print except for pigment, it's a monotype. If there are etched lines incised into the plate, or other textures and matrix adhered to it that can make repeatable impressions, then it's a monoprint. Even if the painting done on the plate changes each time the artist pulls a print, if there is element on the plate that will repeat, regardless of pigment manipulation, it's a monoprint.