A cellular map 
between CW-complexes 
and 
takes 
-skeletons
to 
-skeletons for all 
. Writing 
and 
for the respective 
-skeletons of 
and 
, this condition is
 |
for all nonnegative integers 
.
The contents of the cellular approximation theorem is that, in a certain sense, all maps between CW-complexes can be taken to be cellular.
