subset
set whose elements are all contained in another set
fundamental domain
subset of a space acted upon by a group, which contains exactly one point from each of the orbits
nowhere dense set
mathematical set whose closure has empty interior
cofinal subset
in order theory, a subset 𝑌 of a poset 𝑋 such that for any element of 𝑋, there exists an element of 𝑌 larger than it
proper subset
a proper subset of a set S is a subset of S that is not equal to S