Simple Definition:

Strain is the amount of stress or pressure applied to an object, which can cause it to change shape or size. It is a measure of how much an object stretches, compresses, or twists when a force is applied to it. For example, when you pull on a rubber band, it stretches and gets longer. This change in length is the strain.

Very Simple Definition:

Strain is when something gets longer, shorter, or twisted when you push or pull on it. For example, when you stretch a rubber band and it gets longer, that’s strain.