Defect of an unbounded operator - Deficiency of a linear system - Denjoy-Carleman theorem - Denjoy-Luzin theorem - Differential ideal - Directrix of rational normal scroll - Dirichlet algebra - Dirichlet norm - Dissipative operator - Divisor with normal crossings - Dixmier-Kirillov theorem - Dominant weight - Double curve of an algebraic surface - Dual abelian variety - Dual Kummer surface - Dual Schubert cycle - Dehn-Nielsen theorem - Dehn's algorithm - Dehn's lemma - Decent convergence - Differentiable representation - Differentiable vector - Adjoint of a differential operator - Discrete series - Discretely related groups - Dominant weight - Domination of parabolic pairs - Dual cone - Dual module - Derived functor of inverse limit - Diagonal approximation - Divided polynomial algebra - Dandelin-Gräffe method - Deterministic quantum mechanics - Dade's conjecture - Direct sum conjecture - D'Oliveira's determinantal conjecture