A lens that is concave on one side and convex on the other. If the curvature of the convex part is greater than that of the concave part, the lens is a magnifying glass. If the concave part has greater curvature than the convex part, the lens is a de-magnifying glass.
Inexpensive screw-in close-up lenses are of the meniscus type, this being a good compromise between optical quality (freedom from aberrations) and cost.