A set of points that all lie within the same line are said to be collinear. According to Wolfram Mathworld, three points are collinear if and only if the distance between one point and the line formed by the other two points is equal to zero.
If point A is collinear with points B, C, D and so on, that means that there is a straight line that contains all of these points. Any two points are collinear, because a line can be drawn through them. Three points that are not collinear make up a triangle; however, three points that are collinear create a triangle with an area of zero.