Quadrilaterals
A quadrilateral is a closed 2D figure with four straight sides ("quad" means four and "lateral" means side).
Some properties of quadrilaterals have are:
There are a few special quadrilaterals with specific names based on their properties:

Tip: Based on their properties, all rectangles, squares, and rhombuses are parallelograms, but not all parallelograms are rectangles/squares/rhombuses! Similar idea occurs with rectangles and squares  all squares are rectangles, but not all rectangles are squares.
Triangles
When the base and height of a triangle are given, the area is calculated as: \(Area=\frac{1}{2} \times base \times height\) \(=\frac{1}{2}bh\)

When all the side lengths of the triangle are given, the area is calculated using Heron's Formula: \(Area=\sqrt{s(sa)(sb)(sc)}\), where \(s=\frac{1}{2}(a+b+c)\) 
Circles
Arc  Any connected part between two points on a circle. 
Chord  A line segment whose endpoints lie on the circle (divides a circle into two segments). 
Radius  The straightline segment that connects a centre of the circle to a point on a circle. 
Sector  A figure formed by two radii and an arc (determined by the endpoints of the radii). 
Segment  A figure formed by an arc and chord joining the endpoints of the arc. 
Vertex (Vertices)  A point where two or more line segments meet (i.e. a corner). 
Edge (Edges)  A line segment joining one vertex/corner to another or a line segment where two faces meet. 
Face (Faces)  Any individual flat surface of a 3D figure. 
Surface Area  Total area of the surfaces of a 3D figure. 
Lateral Area  Surface area of a 3D figure, excluding its base and top (when they exist). 
Volume  The amount of 3D space that is occupied/taken up by a 3D figure. 
Example 1