Simple Definition:

Strain is a measure of how much an object deforms or changes shape when a force is applied to it. It is calculated by dividing the change in length or size of the object by its original length or size. Strain can be caused by stretching, squeezing, or twisting an object.

For example, imagine you have a rubber band and you stretch it. The rubber band becomes longer and thinner, and this change in shape is called strain. Another example is squeezing a sponge. When you squeeze it, the sponge becomes smaller and this is also a form of strain.

Very Simple Definition:

Strain is what happens to an object when you pull it or push it. It can change its shape or size. For example, when you stretch a rubber band, it gets longer and thinner. And when you squeeze a sponge, it gets smaller.