average atomic mass 

The weighted average of the atmoic masses of the naturally occuring isotopes of an element.

Suppose you had a box of marbles that contain 2 sizes of marbles. If 25% of the marbles have the massses of 2.00g each and 75% have the masses of 3.00g each then the average atomic mass would look like this.
(25 marbles x 2.00g = 50g)+(75 marbles x 3.00g = 225g) = 275g / 100 = (an average atomic mass of) 2.75g.

average atomic mass by Killa Chris the Great September 29, 2006