Geometrical Terms
4.1 Geometrical Terms – Cheat Sheet
1. Basic Geometrical Terms
| Term | Definition | Example |
|---|---|---|
| Point | An exact location in space, has no size | • |
| Vertex (plural: vertices) | A corner point where two or more lines/edges meet | Corner of a triangle |
| Line | A straight path extending infinitely in both directions | ↔ |
| Plane | A flat surface extending infinitely in all directions | Sheet of paper surface |
| Parallel | Lines that never meet and are always the same distance apart | Railway tracks |
| Perpendicular | Lines that meet at a right angle (90°) | Corner of a square |
| Perpendicular bisector | A line that cuts another line exactly in half at 90° | Midpoint cross line |
| Bearing | Direction given as an angle measured clockwise from north (3-figure format) | 045° |
| Right angle | Exactly 90° | Corner of a rectangle |
| Acute angle | Less than 90° | 45° |
| Obtuse angle | Between 90° and 180° | 120° |
| Reflex angle | Between 180° and 360° | 270° |
| Interior angle | Inside angle of a polygon at a vertex | Triangle interior corner |
| Exterior angle | Angle formed outside a polygon between one side and the extension of another | Supplement of interior angle |
| Similar | Same shape, proportional sides, equal angles | Scale drawings |
| Congruent | Same shape and size | Identical triangles |
| Scale factor | Ratio of lengths between similar figures | 1:3 |
2. Vocabulary of Triangles
| Triangle Type | Properties |
|---|---|
| Equilateral | 3 equal sides, 3 equal angles of 60° |
| Isosceles | 2 equal sides, 2 equal base angles |
| Scalene | All sides and angles different |
| Right-angled | One angle = 90° |
3. Vocabulary of Special Quadrilaterals
| Quadrilateral | Properties |
|---|---|
| Square | 4 equal sides, 4 right angles, diagonals equal and perpendicular |
| Rectangle | Opposite sides equal, 4 right angles, diagonals equal |
| Kite | 2 pairs of adjacent equal sides, 1 pair of equal opposite angles |
| Rhombus | 4 equal sides, opposite angles equal, diagonals perpendicular |
| Parallelogram | Opposite sides parallel and equal, opposite angles equal |
| Trapezium | At least 1 pair of parallel sides |
4. Vocabulary of Polygons
| Polygon | Sides | Notes |
|---|---|---|
| Regular | All sides and angles equal | Regular hexagon |
| Irregular | Sides/angles not all equal | Scalene pentagon |
| Pentagon | 5 sides | Interior sum = 540° |
| Hexagon | 6 sides | Interior sum = 720° |
| Octagon | 8 sides | Interior sum = 1080° |
| Decagon | 10 sides | Interior sum = 1440° |
5. Vocabulary of Solids
| Solid | Description |
|---|---|
| Cube | 6 equal square faces |
| Cuboid | 6 rectangular faces |
| Prism | Two identical parallel faces, other faces rectangles |
| Cylinder | Two identical circular faces connected by curved surface |
| Pyramid | Base is polygon, other faces triangles meeting at apex |
| Cone | Circular base with curved surface to a point |
| Sphere | Perfectly round 3D shape |
| Hemisphere | Half a sphere |
| Frustum | Part of a cone/pyramid cut by a plane parallel to the base |
| Face | Flat or curved surface on a solid |
| Surface | Outer layer/boundary of solid |
| Edge | Line segment where two faces meet |
6. Vocabulary of a Circle
| Term | Definition |
|---|---|
| Centre | Middle point of the circle |
| Radius (radii) | Distance from centre to any point on the circle |
| Diameter | Distance across the circle through the centre (2 × radius) |
| Circumference | Perimeter of the circle |
| Semicircle | Half of a circle |
| Chord | Line joining two points on circumference |
| Tangent | Line touching circle at exactly one point |
| Major arc | Longer arc of circle |
| Minor arc | Shorter arc of circle |
| Sector | Region between two radii and arc |
| Segment | Region between a chord and the arc it subtends |