Monotype also refers to a particular printmaking technique where the artist paints ink directly onto a plate, and then transfers the ink onto paper by running the plate through a press. The majority of the ink is removed, and, as a result, this technique produces only one print (and thus the mono portion of the name).
Monotypes vaguely resemble paintings, but tend to be more fluid and expressive. They are generally monochromatic, but one can also create color images by using multiple layers of colored ink. The intensity of any given color (or the level of shading in a monochromatic piece) is determined by the thickness of the ink in that spot.
There are two fundamental kinds of monotypes, which are differentiated by the way they are created. For an additive monotype, one applies the ink to a clean plate directly. For a subtractive monotype, one covers the entire plate with a thick layer of ink first -- thus establishing the deepest value -- and then wipes away as needed in order to create lighter shades. One can also use turpentine or mineral spirits to dissolve the ink and create wash effects.