Phrase. Denotes a system of thought that predominantly considers things in an "either, or", "right, wrong", "black, white" way, ignoring any subtleties or consideration of third or more alternatives.
In philosophy, this is know as a "bifurcation fallacy".
People who habitually think in this way are usually fairly unintelligent and unimaginative.
G.W. Bush: "You're either with us, or your with the terrorists".
Me: "Wow, Bush can only seem to understand the world through binary thinking..."