A point is an exact position with no size. A line extends without ending; a line segment has two endpoints (its vertices). Two lines are parallel ($\parallel$) if they never meet and stay the same distance apart; they are perpendicular ($\perp$) if they cross at a right angle ($90°$). A perpendicular bisector cuts a segment exactly in half at a right angle. A plane is a completely flat surface extending in two dimensions, and a bearing is a direction measured clockwise from north.

• Acute: between $0°$ and $90°$
• Right angle: exactly $90°$
• Obtuse: between $90°$ and $180°$
• Reflex: between $180°$ and $360°$

A shape's interior angles are on the inside; its exterior angles are formed by extending one side outward.

Triangles: equilateral (all sides/angles equal), isosceles (two sides/angles equal), scalene (no sides equal), right-angled (one $90°$ angle).

Quadrilaterals: square, rectangle, rhombus (all sides equal, opposite sides parallel), parallelogram (opposite sides parallel and equal), kite (two pairs of adjacent equal sides), trapezium (one pair of parallel sides).

Two shapes are similar if one is an enlargement of the other (same angles, sides in the same ratio — this ratio is the scale factor). Two shapes are congruent if they are exactly identical in size and shape (angles equal AND corresponding sides equal).

A polygon is a closed 2D shape with straight sides — regular if all sides and angles are equal, irregular otherwise. Named by side count: pentagon (5), hexagon (6), octagon (8), decagon (10). A net is a 2D shape that folds up into a 3D solid. Key solids: cube, cuboid, prism (uniform cross-section along its length), cylinder, pyramid, cone, sphere, hemisphere (half a sphere), and frustum (a cone or pyramid with the top sliced off parallel to its base). A solid's face is a flat or curved outer surface, its edge is where two faces meet, and its vertex is a corner point.

• Centre — the middle point, equidistant from every point on the circle.
• Radius (plural radii) — centre to edge; diameter — edge to edge through the centre ($= 2 \times$ radius).
• Circumference — the distance around the circle; semicircle — exactly half.
• Chord — a straight line joining two points on the circle (not necessarily through the centre).
• Tangent — a straight line that touches the circle at exactly one point.
• Arc — part of the circumference; major arc (the longer one) and minor arc (the shorter one).
• Sector — a "pizza slice" region bounded by two radii and an arc.
• Segment — a region bounded by a chord and an arc.

(a) A cone has its top sliced off by a cut parallel to its base. Name the resulting solid. (b) A circle has two radii drawn from the centre to its edge, enclosing part of the circle's area between them. Name the shaded region.

1. A cone with its top removed by a cut parallel to the base is a frustum.
2. A region enclosed by two radii and the arc between them is a sector.