Hooke's Law

Extension/compression is directly proportional to the force applied
or
Strain is directly proportional to stress

(Only if within the limit of proportionality)

Spring constant / Force constant

Measure of elasticity of an object

$$\begin{align*} F &= kx \\ \text{load} &= \text{spring constant} \cdot \text{extension} \end{align*}$$

Unit of $k$ = $\mathrm{N\,m^{-1}}$

Large spring constant = stiff
Small spring constant = elastic