For questions related to the convex hull of a set, often (but not limited to) referring to the feasible region in optimization.
The convex hull of a set X is the smallest convex set that contains X.
Alternate and more formal definitions available.
The Elastic Band analogy demonstrates this below for a finite set in 2D. Surround the points with the smallest elastic band that contains the points. The result is a convex set itself (blue band) and is the smallest convex set possible that contains all 10 points. All points inside this set (the blue band) comprise the convex hull for these 10 points. Source, "Convex hull of a finite set: elastic-band analogy" image from public domain.