Lines ab and cd are skew lines.
Skew lines are straight lines in a three dimensional form which are not parallel and do not cross.
An example of skew lines are the sidewalk in front of a house and a line running across the top edge of a side of a house.
Math. two or more lines that lie in different planes, are not parallel, and do not intersect
Straight lines that are not in the same plane and do not intersect.