A line has one dimension. It is represented by a line with two arrowheads (↔), but it extends without end. Through any two points, there is exactly one line. You can use any two points on a line to name it.
A plane has two dimensions. It is represented by a shape that looks like a floor or a wall, but it extends without end. Through any three points not on the same line, there is exactly one plane. You can use three points that are not all on the same line to name a plane.
Words that do not have formal definitions, but there is agreements about what they mean. Point, line, and plane are all examples.
line segment (or segment)
(written with a line, − , over AB) consists of the endpoints A and B and all points on line AB that are between A and B. Line segment AB can also be named line segment BA.
(written with an arrow, →, over AB) consists of the endpoint A and all points on line AB that line on the same side of A as B. Ray AB and Ray BA are DIFFERENT rays.
If point C lies on line AB between A and B, then ray CA and ray CB are opposite rays. Opposite rays are collinear.
The set of points the geometric figures have in common, when two or more geometric figures intersect.
Disntance between points A and B, written as AB, is the absolute value of the difference of the coordinates of A and B. AB = |x₂ - x₁|
Point B is between point A and point C iff (if and only if) point B is collinear to point A and point C.
Segment Addition Postulate
If B is between A and C, then AB + BC = AC. If AB + BC = AC, then B is between A and C.
A point, ray, line, line segment, or plane that intersects the segment at its midpoint.
The coordinates of the midpoint of a segment are the averages of the x-coordinates and of the y-coordinates of the endpoints. If A (x₁, y₁) and B (x₂, y₂) are points in a coordinate plane, then the midpoint M of segment AB has coordinates: [(x₁+x₂)/2 , (y₁+y₂)/2]