Geometric invariant theory studies the construction of moduli spaces / moduli stacks in terms of quotients / action groupoids.
This is the geometric aspect (Isbell dual aspect) of invariant theory.
David Mumford, John Fogarty, Frances Clare Kirwan, Geometric invariant theory, Ergebnisse der Mathematik und ihrer Grenzgebiete (2) 34, Springer-Verlag
David Mumford, Stability of projective varieties, L’Enseignement Mathématique 23 (1977) doi
David J. Swinarski, Geometric Invariant Theory and Moduli Spaces of Maps (pdf)
Jürgen Hausen, A generalization of Mumford’s geometric invariant theory (pdf)