在连续介质力学里，应力定义为单位面积所承受的作用力。以公式标记为

其中，表示应力；表示在方向的施力；表示在方向的受力面积。

假设受力面积与施力方向正交，则称此应力分量为正向应力，如图1所示的、、，都是正向应力；假设受力面积与施力方向互相平行，则称此应力分量为剪应力（shear stress），如图1所示的、、、、、，都是剪应力。

「内应力」指组成单一构造的不同材质之间，因材质差异而导致变形方式的不同，继而产生的各种应力。

采用国际单位制，应力的单位是帕斯卡（Pa），等于1牛顿／平方公尺。应力的单位与压强的单位相同。两种物理量都是单位面积的作用力的度量。通常，在工程学里，使用的单位是megapascals（MPa）或gigapascals（GPa）。采用英制单位，应力的单位是磅力／平方英寸（psi）或千磅力／平方英寸（ksi）。

In continuum mechanics, stress is a physical quantity that expresses the internal forces that neighboring particles of a continuous material exert on each other, while strain is the measure of the deformation of the material. For example, when a solid vertical bar is supporting a weight, each particle in the bar pushes on the particles immediately below it. When a liquid is in a closed container under pressure, each particle gets pushed against by all the surrounding particles. The container walls and the pressure-inducing surface (such as a piston) push against them in (Newtonian) reaction. These macroscopic forces are actually the average of a very large number of intermolecular forces and collisions between the particles in those molecules.