point A specific position in space. line A straight set of points. plane A flat surface. angle The space between two intersecting lines or rays. apex The topmost point. base The bottom of a figure. degree A unit for measuring angles. vertex The point where two lines meet; a corner. acute angle An angle which is less than 90 degrees. obtuse angle An angle which is greater than 90 degrees. right angle An angle which is exactly 90 degrees. bisect To divide an angle into two equal parts. congruent Equal. intersection The point where two lines cross. symmetry Describes when a figure has exactly the same parts on either side of a line. line of symmetry The line which divides a figure such that on either side of the line, parts of the figure are exactly the same. parallel Describes two lines which are equally distant from each other at every point. perpendicular Describes when the angle between two lines or planes is a right angle. quadrilateral Any four-sided figure. trapezoid A quadrilateral in which exactly two sides are parallel. parallelogram A quadrilateral in which opposite sides are parallel to each other. rhombus A parallelogram in which every side is equal. square A parallelogram in which every side is equal and every angle is a right angle. rectangle A parallelogram in which every angle is a right angle. triangle Any three-sided figure. isosceles triangle A triangle in which two sides and two angles are equal. equilateral triangle A triangle in which all three sides and all three angles are equal. scalene triangle A triangle in which no sides or angles are equal.