An experimental
design is orthogonal if each
factor can be evaluated independently of
all the other factors. In a two level factorial
design, this is achieved by
matching each level of each factor with
an equal number of each level of the other
factors.
For example, in the array the '+1' level
of Factor A (runs 2 and 4) is matched with
one instance of Factor B at '1' and one
at '+1'. If any two columns are compared,
the same thing will be found for both factor
levels.
Treatment 
A 
B 
AB
+ C 
1 
1 
1 
+1 
2 
+1 
1 
1 
3 
1 
+1 
1 
4 
+1 
+1 
+1 
The term 'orthogonal array' is often used
in the context of Taguchi
designs; 'Taguchi Orthogonal
Arrays'.
