**
Acute Angle
** -
An angle whose measure is less than 90 degrees.

**
Adjacent Angle
** -
Angles that share a vertex, one side, and no interior
points.

**
Alternate Exterior Angles
** -
Angles created when a transversal intersects with two
lines. Alternate exterior angles
lie on opposite sides of the transversal, and on the exterior of
the space between the two lines.

**
Alternate Interior Angles
** -
Angles created when a transversal intersects with two
lines. Alternate interior angles lie on
opposite sides of the transversal, and on the interior of the space between
the two lines. That is, they lie between the two lines that intersect with the
transversal.

**
Angle
** -
A geometric figure consisting of the union of two
rays that share a common endpoint.

**
Angle Bisector
** -
A ray that shares a common vertex with an angle, lies within the
interior of that angle, and creates two new angles of equal measure.

**
Angle Trisector
** -
A ray, one of a pair, that shares a common vertex with an angle,
lies within the interior of that angle, and creates, with its partner, three
new angles of equal measure. Angle trisectors come in pairs.

**
Complementary Angles
** -
A pair of angles whose measures sum to 90 degrees. Each angle in the pair
is the other's complement.

**
Congruent
** -
Of the same size. Angles can be congruent to other angles and
segments can be congruent to other
segments.

**
Corresponding Angles
** -
A pair of angles created when a transversal intersects with two
lines. Each angle in the pair is on the same
side of the transversal, but one is in the exterior of the space created
between the lines, and one lies on the interior, between the lines.

**
Degree
** -
A unit of measure for the size of an angle. One full rotation is equal to
360 degrees. A right angle is 90 degrees. One degree equals

radians.

**
Exterior Angle
** -
The larger part of an angle. Were one of the rays of an angle to be rotated
until it met the other ray, an exterior angle is spanned by the greater rotation
of the two possible rotations. The measure of an exterior angle is always
greater than 180 degrees and is always 360 degrees minus the measure of the
interior angle that accompanies it. Together, an interior and exterior
angle span the entire plane.

**
Interior Angle
** -
The smaller part of an angle, spanned by the space between the rays that
form an angle. Its measure is always less than 180 degrees, and is equal to 360
degrees minus the measure of the exterior angle.

**
Midpoint
** -
The point on a segment that lies exactly
halfway from each end of the segment. The distance from the endpoint of a
segment to its midpoint is half the length of the whole segment.

**
Oblique
** -
Not perpendicular.

**
Obtuse Angle
** -
An angle whose measure is greater than 90 degrees.

**
Parallel Lines
** -
Lines that never intersect.

**
Parallel Postulate
** -
A postulate which states that given a point
not located on a line, exactly

*one* line passes through the point that is
parallel to original line.

Figure %: The parallel postulate

**
Perpendicular
** -
At a 90 degree angle. A geometric figure (line, segment, plane, etc.) is
always perpendicular *to* another figure.

**
Perpendicular Bisector
** -
A line or
segment that is perpendicular to a
segment and contains the midpoint of that segment.

**
Radian
** -
A unit for measuring the size of an angle. One full rotation is equal to

2*Π*
radians. One radian is equal to

degrees.

**
Ray
** -
A portion of a line with a fixed
endpoint on one end that extends without bound in the other direction.

**
Right Angle
** -
A 90 degree angle. It is the angle formed when perpendicular lines or
segments intersect.

**
Segment Bisector
** -
A line or
segment that contains the midpoint of a
segment.

**
Straight Angle
** -
A 180 degree angle. Formed by two
rays that share a common vertex
and point in opposite directions.

**
Supplementary Angles
** -
A pair of angles whose measures sum to 180 degrees. Each angle in the
pair is the other's supplement.

**
Transversal
** -
A line that intersects with two other lines.

**
Vertex
** -
The common endpoint of two rays at
which an angle is formed.

**
Vertical Angles
** -
Pairs of angles formed where two lines
intersect. These angles are formed by rays pointing in opposite directions,
and they are congruent. Vertical angles come in pairs.

**
Zero Angle
** -
A zero degree angle. It is formed by two rays that share a vertex
and point in the same direction.