Excerpt from Implicitly Representing Arrangements of Lines or Segments
What do we mean by which face? We want a complete description of the face containing p in terms of the lines in L, that is, which lines of L appear along the boundary of that face in, say, counterclockwise order. In some cases we may be satisfied with a simpler property of the face, such as its intersection with a line, its leftmost vertex, the edge below p, or the number of lines on the face boundary.