Let 
and 
be CW-complexes and let 
(respectively 
) denote the 
-skeleton of 
(respectively 
). Then a continuous map 
is said to be cellular if it takes 
-skeletons to 
-skeletons for all 
, i.e, if
 |
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.
