angle noun A figure formed by two rays that start at the same point, or the measure of the turn between them. The two rays form a right angle. + vertex noun The point where two or more lines or edges meet, such as the corner of an angle or shape. Each triangle has three vertices, the points where its sides intersect. + parallel adjective Describing straight lines in the same plane that never meet, however far they extend. Opposite sides of a rectangle are parallel. + perpendicular adjective Positioned at an exact right angle, ninety degrees, to another line or surface. The carpenter checked that the wall was perpendicular to the floor before installing shelves. + polygon noun A closed, flat shape with three or more straight sides. A triangle is a polygon with three sides. + triangle noun A geometric shape with three straight sides and three angles. The teacher drew a triangle on the board to explain its three angles. + quadrilateral noun A polygon with exactly four sides and four angles. Squares, rectangles, and trapezoids are all examples of a quadrilateral. + hypotenuse noun The longest side of a right triangle, located opposite the right angle. By the Pythagorean theorem, the square of the hypotenuse equals the sum of the squares of the other two sides. + congruent adjective Having exactly the same size and shape, so that one figure matches another perfectly. The two triangles are congruent because their corresponding sides and angles are equal. + similar adjective Alike in one or more ways without necessarily being identical. The two coats are similar in color but different in length. + symmetry noun The quality of having balanced, matching proportions on either side of a dividing line or point. The butterfly's wings show near-perfect symmetry in their color and pattern. + radius noun A line segment from a circle's center to its boundary, or the length of that segment. The radius of a circle with diameter 12 centimeters is 6 centimeters. + diameter noun A line segment across a circle through its center, or the length of that segment. Draw a diameter through the center of the circle. + circumference noun The distance around a circle. We wrapped a string around the lid to estimate its circumference. + perimeter noun The total distance around the boundary of a flat shape. The square has sides of 4 centimeters and a perimeter of 16 centimeters. + area noun The amount of two-dimensional space inside a region or across a surface. A rectangle that is 3 meters by 4 meters has an area of 12 square meters. + volume noun The degree of loudness of a sound, or the control that adjusts it. Please turn down the volume; the music is too loud for a library. + coordinate plane noun A flat surface defined by two perpendicular number lines, used to locate points with ordered pairs. Students plotted each point on the coordinate plane using its x and y values. + line segment noun A straight part of a line that has two distinct endpoints and a definite length. The diagram showed a line segment connecting point A to point B. + ray noun A narrow beam of light, heat, or other radiation traveling in a straight line from its source. A single ray of sunlight broke through the clouds and lit up the valley below. + obtuse adjective Slow to understand things that are obvious to other people; also, an angle greater than 90 degrees. He seemed deliberately obtuse, ignoring every hint that his plan wouldn't work. + acute adjective Very serious, severe, or intense, especially describing a problem, feeling, or sensation. The refugees faced an acute shortage of clean drinking water. + right angle noun An angle of exactly ninety degrees, formed where two lines meet perpendicularly. The corners of a standard sheet of paper each form a right angle. + equilateral adjective Having all sides of equal length, especially describing a triangle. An equilateral triangle has three equal sides and three sixty-degree angles. + isosceles adjective Describing a triangle that has exactly two sides of equal length and two equal angles. The students measured the isosceles triangle to confirm that two of its sides matched. + scalene adjective Describing a triangle in which all three sides and all three angles are of different measures. The teacher drew a scalene triangle on the board to contrast it with an equilateral one. + diagonal adjective Running in a straight, slanting direction rather than horizontally or vertically, especially between two opposite corners. The bishop in chess can only move along a diagonal line across the board. + tangent noun A straight line that touches a curve at one point without crossing into it, especially in geometry. The engineer calculated the tangent to the circle at the exact point where the road curved away. + bisector noun A line, ray, or plane that divides an angle or line segment into two equal parts. The teacher drew a bisector through the angle to split it into two equal halves. + transformation noun A complete and often dramatic change in form, appearance, or character. The caterpillar's transformation into a butterfly amazed the young students. +