The convex hull of a set \(X\) in an affine space over the reals is the smallest convex set that contains \(X\). When the points are two dimensional, the convex hull can be thought of as the rubber band around the points of \(X\).

As per Wikipedia, a convex set is the smallest affine space closed under convex combination.

A convex combination is a linear combination where all the coefficients are greater than 0 and all sum to 1.