Terminology (Leaving Cert Mathematics): Revision Notes

Terminology

This list serves as a glossary of essential terms in geometry, providing a foundation for understanding and applying geometric concepts.

Geometric Terminology

• Acute Angle: An angle measuring less than 90°.
• Adjacent Angles: Two angles that share a common side and vertex but do not overlap.
• Altitude: A perpendicular segment from a vertex of a triangle to the opposite side (or its extension).
• Angle: Formed by two rays with a common endpoint (vertex).
• Arc: A part of the circumference of a circle.
• Bisector: A line or segment that divides another segment or angle into two equal parts.
• Centre: The fixed point equidistant from all points on a circle or sphere.
• Chord: A line segment joining two points on a curve, specifically on a circle.
• Collinear Points: Points that lie on the same straight line.
• Complementary Angles: Two angles whose measures add up to 90°.
• Concave Polygon: A polygon with at least one interior angle greater than 180°.
• Convex Polygon: A polygon where all interior angles are less than 180°.
• Concurrent Lines: Three or more lines that intersect at a single point.
• Diameter: A chord that passes through the centre of a circle, the longest chord in a circle.
• Equilateral Triangle: A triangle with all sides of equal length and all angles equal.
• Exterior Angle: An angle formed outside a polygon when one side is extended.
• Face: A flat surface on a three-dimensional solid.
• Height: The perpendicular distance from the base to the opposite side or vertex in a geometric figure.
• Hypotenuse: The longest side of a right-angled triangle, opposite the right angle.
• Interior Angle: An angle formed between two adjacent sides of a polygon inside the figure.
• Isosceles Triangle: A triangle with at least two sides of equal length.
• Line Segment: A part of a line defined by two endpoints.
• Midpoint: The point on a line segment that divides it into two equal parts.
• Obtuse Angle: An angle measuring more than 90° but less than 180°.
• Parallel Lines: Lines in the same plane that never intersect.

• Parallelogram: A quadrilateral with opposite sides parallel and equal in length.
• Perimeter: The total length of the boundary of a polygon.
• Perpendicular Lines: Lines that intersect to form a 90° angle.
• Plane: A flat, two-dimensional surface that extends infinitely in all directions.
• Point: A location in space with no size or dimension.
• Polygon: A closed, two-dimensional shape formed by straight line segments.
• Radius: A line segment from the centre of a circle to any point on its circumference.
• Ray: A part of a line that starts at one point and extends infinitely in one direction.
• Rectangle: A quadrilateral with four right angles.
• Reflex Angle: An angle greater than 180° but less than 360°.
• Rhombus: A quadrilateral with all sides of equal length.
• Right Angle: An angle measuring exactly 90°.
• Scalene Triangle: A triangle with all sides of different lengths.
• Sector: A region of a circle bounded by two radii and their intercepted arc.
• Tangent (Circle): A line that touches a circle at exactly one point.
• Trapezium (Trapezoid): A quadrilateral with one pair of parallel sides.
• Triangle: A three-sided polygon.

• Vertex: A point where two or more lines, rays, or edges meet.