Geometry Fundamentals

Points, lines, planes, and space are the building blocks of geometry.
Point: Represented by a dot. It has no length or width and is just a placeholder.
Line: Has no width, but its length extends infinitely in two directions. A line does not have any end points.
Ray: Begins at one point and extends infinitely in one direction. A ray has exactly one end point.
Segment: A part of a line or ray that is between two points. A segment has two end points.
Plane: A flat surface that extends infinitely.
Space: A set of all points in three dimensions.

Midpoint: The exact middle of a segment.
Segment Bisection: When a segment is bisected, its halves are equal (congruent).

Intersecting Lines: Lines that cross each other if they are not parallel.
Parallel Lines: Lines that extend infinitely without crossing. Parallel lines are always coplanar.
Skew Lines: Lines that will never intersect in space and are not parallel.
Perpendicular Lines: Intersecting lines that form right angles.

Angle: Formed by two rays that share the same endpoint.
Angle Bisector: A ray that splits an angle, forming two congruent adjacent angles.

Acute Angle: An angle that measures less than $90^\text{degrees}$ .
Right Angle: An angle that measures exactly $90^\text{degrees}$ . A right angle is designated by placing a small square at the vertex of the angle.
Obtuse Angle: An angle that measures more than $90^\text{degrees}$ but less than $180^\text{degrees}$ .
Straight Angle: An angle that measures exactly $180^\text{degrees}$ . A straight angle looks just like a line.

Adjacent Angles: Coplanar angles that have a common vertex and a common side.
Complementary Angles: Two angles whose measures add up to $90^\text{degrees}$ .
Supplementary Angles: Two angles whose measures add up to $180^\text{degrees}$ .
Vertical Angles: Two angles that occur opposite each other when two lines intersect. Vertical angles are congruent.