Lines:

A line is the path created by a moving point.  A length of a straight line is the shortest distance among two nonadjacent points and is made up of collinear points.  A line segment is a portion of a line.  A ray is an infinite set of collinear points extending from one end point to infinity. A group of points is noncollinear if the points are not contained in a line.

Two or more straight lines are parallel while they are coplanar (contained in the similar plane) and do not intersect; that is, while they are an equal distance apart at every point.