If you take these circles which are exactly the same size,
and measure that line across the one on the right which is
called its DIAMETER,




you'll find that it measures exactly 1 inch across.  That means 
its DIAMETER is 1 inch.

Now suppose we were to cut that first circle and roll it out 
flat, or straight, then it would look like this line:



Now if you measure that line you'll find that it measures about
3.14 inches.  That's pi or p. 

That's what is meant by the circumference of a circle. The circumference
is how long the circle would be if you cut it and straightened it out.

The above demonstrates the formula

C = pd

which says that the circumference of any circle is always a little more
that 3 times as far around it as it is across it (its diameter).  And that
"a little more than three" is what we call the number "pi" and abbreviate 
with the Greek letter p.  

{Remark: It's too bad that pi didn't come out to be a nice number like 3.
In the Bible, 1 Kings 7:23 reads:

"And he made a molten sea ten cubits from the one brim to the other it was round all about...and a line 
of thirty cubits. 

So they must have thought pi was exactly 3 way back then, but we know better now -- that it is a little more than 3.]

Edwin