The basic definition of a circle: A round two dimensional plane.
Diameter: The distance across a circle through its centre. (usually represented by d)
It is NOT this:
It is this:
Radius (Plural is Radii.): The distance from the centre of a circle to its outer edge (usually represented as r)
(Some random fact: If you measure from the centre to the outer edge of a "perfect" circle all around, the measurements will be virtually the same. This is what fully defines a circle, a round, two-dimensional plane that measured from the centre to the outer edge, the measurements would be equal in distance.)
We can now safely state that 2r=d. This means that 2 radii are needed to make the diameter of a circle.
Circumference: The perimeter around the circle, it is usually represented as C. (this is what I mean by "outer edge".)
To figure out the circumference, multiply the diameter by Pi (π).
Pi is approximate to 3.14.
Some circumference formulas:
(I'll also throw in on how to figure out the area.)
Some equivalents to a rectangle or parallelogram:
Width/Base: Half of the circumference
How to figure out the area of a rectangle or parallelogram: Length x Width (or Base x Height)
Enough Parallelogram talk, to figure out the area of a circle is:
Pi radius square.