Sentence:  Definition If "E" is a non-empty set then by an order on "E" we mean a <WSD>binary relation</WSD> on "E" that is reflexive, anti-symmetric, and transitive. 
Meaning of the word marked with <WSD> tags:  A subset of the Cartesian product "A"×"A" (the set of ordered pairs ("a", "b") of elements of "A").