Sentence:  Comparing the use of <WSD>convex envelopes</WSD> for bilinear and trilinear forms in building convex approximations for MINLPs motivated the study in [6], and comparisons involving more general functional forms motivate the present article. 
Meaning of the word marked with <WSD> tags:  For a given set S\subseteq\mathbb{R}^n and real-valued function "f" defined on the convex hull conv("S"), the highest-valued convex function that underestimates or equals "f" over "S".