Stress

Forces in the earth are usually termed as stress. Stress is like a force, but there are important differences:

1. Stress is force per unit area, like pressure.
2. Stress is defined to be at mechanical equilibrium (all forces balanced). Thus stress is represented by a pair of opposing vectors (a single vector is called a traction).
3. Stresses are in general not perpendicular to a plane in the earth, so there are two components, normal stress, and shear stress.