Sentence:  The orthogonal convex <WSD>hull</WSD> of an orthogonal polygon is the smallest orthogonally convex polygon that encloses the original polygon. 
Meaning of the word marked with <WSD> tags:  The smallest set that possesses a particular property (such as convexity) and contains every point of "A"; slightly more formally, the intersection of all sets which possess the specified property and of which "A" is a subset.