commutative diagram
where the commutativity of the diagram means 
, as both these compositions arise from paths from A to Y
- decreasing sequence
- Dedekind cut
- Dedekind, (Julius Wilhelm) Richard
- deduction
- deficient number
- definite integral
- deformation
- degenerate
- degenerate conic
- degenerate quadric
- degree
- degree
- degree
- degree
- degree-genus formula
- degrees of freedom
- degrees of freedom
- deka‐
- del
- Delian (altar) problem
- delta
- delta function
- demography
- De Moivre, Abraham
- De Moivre's Theorem