What is "strain"

Basics about the term "strain"

The strain is a measure of the relative change in length.
"Relative change in length" means the increase or decrease of the length Δl relative to the original length l0:
The formula for the strain is ε.

The elongation ε = Δl / 10 is positive in the case of an increase in the original strain (and is referred to as "stretching"), or is negative (and is referred to as "compression")