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Definition of Terms
Term		Definition
Altitude		An altitude of a triangle is a line segment connecting a vertex to the line containing the opposite side and perpendicular to that side.
Angle Bisector		An angle bisector is a ray that cuts the angle exactly in half, making two equal angles.
Centroid		The centroid of a triangle is the point where the three medians meet. This point is the center of mass for the triangle. If you cut a triangle out of a piece of paper and put your pencil point at the centroid, you could balance the triangle.
Circle		A circle is the set of all points in a plane that are equidistant from a given point in the plane, which is the center of the circle.
Circumcenter		The circumcenter of a triangle is the point where the three perpendicular bisectors meet. This point is the same distance from each of the three vertices of the triangles.
Concurrent		When three or more lines meet at a single point, they are said to be concurrent. In a triangle, the three medians, three perpendicular bisectors, three angle bisectors, and three altitudes are each concurrent.
Congruent		Two figures are congruent if all corresponding lengths are the same, and if all corresponding angles have the same measure. Colloquially, we say they "are the same size and shape," though they may have different orientation. (One might be rotated or flipped compared to the other.)
Diameter		A circle's diameter is a segment that passes through the center and has its endpoints on the circle.
Equilateral Triangle		An equilateral triangle is a triangle with three equal sides.
Hypotenuse		The hypotenuse in a right triangle is the side of the triangle that is opposite to the right angle.
Incenter		The incenter of a triangle is the point where the three angle bisectors meet. This point is the same distance from each of the three sides of the triangle.
Isosceles Triangle		An isosceles triangle is a triangle with two equal sides.
Kite		A kite is a quadrilateral that has two pairs of adjacent sides congruent (the same length).
Line		A line has only one dimension: length. It continues forever in two directions (so it has infinite length), but it has no width at all. A line connects two points via the shortest path, and then continues on in both directions.
Median		A median is a segment connecting any vertex of a triangle to the midpoint of the opposite side.
Midline		A midline is a segment connecting two consecutive midpoints of a triangle.
Obtuse triangle		An obtuse triangle is a triangle with one angle more than 90�.
Orthocenter		The orthocenter of a triangle is the point where the three altitudes meet, making them concurrent.
Parallelogram		A parallelogram is a quadrilateral that has two pairs of opposite sides that are parallel.
Quadrilateral		A quadrilateral is a polygon with exactly four sides.
Right Triangle		A right triangle is a triangle with one right (90�) angle.