Sentence:  The function f(x,y)=xy is <WSD>bilinear</WSD>, because f(ax+bu,y)=af(x,y)+bf(u,y) and f(x,ay+bv)=af(x,y)+bf(x,v) for all values. 
Meaning of the word marked with <WSD> tags:  Linear (preserving linear combinations) in each variable.