Uh... sorry, not sure if you're being facetious or not.

But to answer anyway: It's combining several "arms" in a cohesive whole, stronger than the sum of its parts. In Alexander's case it was the combination of his heavy cavalry (Companions, Thessalians) and heavy infantry (Phalangites, hoplites, hypaspistai) and light infantry (Agrianians, Kretans, Macedonians) which together made for a very effective army.

The successors lacked his cavalry strength in particular.