1. Perpendicular Lines
Definition: Two lines that intersect to form right angles.
Note: The symbol  means “is perpendicular to”

2. Perpendicular Bisector of a Segment
Definition A line, segment, or ray that is perpendicular to the segment at its midpoint

3. Parallel Lines Definition Coplanar lines that do not intersect
Note: The symbol || or // means “is parallel to”

4. Skew Lines
Definition Non-coplanar; they are not parallel and do not intersect

5. Parallel Planes Definition Planes that do not intersect
Plane ABC // plane EFG

6. Transversal
Definition A line that intersects two or more coplanar lines at distinct points
Line r is the transversal

7. Alternate Interior Angles
Definition Nonadjacent interior angles that lie on opposite sides of the transversal

8. Alternate Exterior Angles
Definition Nonadjacent exterior angles that lie on opposite sides of the transversal

9. Same Side Interior Angles
Definition Interior angles that lie on the same side of the transversal

10. Corresponding Angles
Definition Angles that lie on the same side of the transversal and in corresponding positions