open set
set that does not contain any of its boundary points
empty set
mathematical set containing no elements
open ball
mathematical ball that excludes its bounding sphere
clopen set
subset that is both open and closed
exterior
term in topology
Mittag-Leffler star
given a complex-analytic function, a star-shaped region in the complex plane obtained by attempting to extend that function along rays emanating from a given point
domain
connected open subset of a finite-dimensional vector space
interior
given a subset S of a topological space X, the biggest set of points in S not part of the boundary of S