Any design that exists solely as small lines. Generally characterized by a lack of shading or fill.
Sometimes an initial session is all linework to make sure the piece can accurately be worked on at a later date.
Sometimes the piece is just linework.
Linework can be useful to fill space without the weight of blackwork, it can serve as a useful texture layer by blasting over existing work, or it can be used to lay the groundwork for future blackwork to go over it.