O.K., Let's be honest, periodicity is not a word that you will use everyday.  
In fact, you probably could go your whole life and never need it.  But it does
a great job of describing the repeating patterns that you find in the periodic
table.
 Periodicity  
(the repeating pattern of the periodic table)
This graph
shows the
amount of
energy it takes
to ionize each
element (listed
by atomic
number)
This graph shows the atomic
radius of each atom by atomic
number