Two words engineers use almost interchangeably in casual conversation — and never interchangeably in an equation. One is a cause. One is an effect. They don't even share units.
Pull on a steel rod and two different things happen at once: the material develops an internal resistance to being pulled apart, and the rod actually gets a little longer. The first of those is stress. The second is strain. They rise and fall together, which is exactly why people start treating them as one idea — but mixing them up is the kind of mistake that turns into a wrong answer the moment a real formula is involved.