quotient stack
Artin stack associated to a scheme X with group action by an affine smooth group scheme G, whose object over a scheme T is a G-torsor P→T with an equivariant map P→X, whose morphisms are G-torsor morphisms compatible with the equivariant maps to X
en-GB/ˈkwəʊʃənt ˈstæk/
en-US/ˈkwoʊʃənt ˈstæk/