Monotype: A single impression, not repeatable. A unique print where the image must be re-created each time, there is no re-usable printing matrix.
(Rosemary Simmons, Dictionary of Printmaking Terms, A & C Black, London.)

Allyson has been working in the medium of monotype since 1987. As the name suggests, a monotype is a unique image, a one-off that cannot be repeated, unlike other methods of printmaking where the image can be reproduced many times. Allyson creates an image using thin oil-based ink painted onto a piece of polished thin-gauge metal. The image is then covered with a piece of damp, cotton-rich paper. Pressure is then applied through a press, which transfers the image on the inked metal to the surface of the paper. The paper is then peeled off the plate to reveal the image.

More infomation:
Tate glossary
The history of monoprints
Wikipedia definition