What Exactly Is an Interaction/moderation Effect

What exactly is an interaction/moderation effect?

A) When a third variable (X₁) and an independent variable (X₂) affect the dependent variable (Y) simultaneously
B) When a third variable (X₁) reduces the effect of an independent variable (X₂) on the dependent variable (Y)
C) When a third variable (X₁) affects the relation between an independent variable (X₂) and the dependent variable (Y)
D) When a third variable (X₁) affects an independent variable (X₂) but not the dependent variable (Y)