Definition of line art:
(n) :
Images that consist of distinct straight and curved lines placed against a (usually plain) background, without gradations in shade (darkness) or hue (color).
(n) :
(printing, graphic design) Artwork to be reproduced with one solid color, contrasted with artwork having tone or shading (halftones), or full color (color separations).
(n) :
The outlines of a drawing as opposed to color, shading, and texture.