在数学中，内乘（interior product，或译内积）是光滑流形上的微分形式外代数上一个次数为 −1 导子，定义为微分形式与一个矢量场的缩并。从而如果 X 是流形 M 上一个矢量场，那幺

是将一个 p-形式 ω 映为 (p−1)-形式 i_Xω，由性质

所定义，对任何矢量场 X₁,..., X_p−1。本质上来说，内乘可以定义在矢量空间与外代数上，即只与流形的一点有关。

内乘也称为内乘法（interior 或 inner multiplication），或内导数（inner derivative 或 derivation）。

In mathematics, the interior product or interior derivative is a degree −1 antiderivation on the exterior algebra of differential forms on a smooth manifold. The interior product, named in opposition to the exterior product, is also called interior or inner multiplication, or the inner derivative or derivation, but should not be confused with an inner product. The interior product ι_Xω is sometimes written as X ⨼ ω.