Define E, G, K and Poisson's ratio

Modulus of elasticity (Young's modulus, $E$)

The ratio of normal (tensile or compressive) stress to the corresponding linear strain, within the elastic limit:

$$E = \frac{\text{normal stress}}{\text{linear strain}} = \frac{\sigma}{\varepsilon}$$

For mild steel, $E \approx 2\times10^5\ \text{N/mm}^2$.

Modulus of rigidity (shear modulus, $G$)

The ratio of shear stress to shear strain within the elastic limit:

$$G = \frac{\tau}{\phi}$$

where $\phi$ is the shear strain (angular distortion in radians).

Bulk modulus ($K$)

The ratio of the uniform (hydrostatic) stress applied on all faces to the resulting volumetric strain, within the elastic limit:

$$K = \frac{p}{\delta V/V}$$

Poisson's ratio ($\nu$ or $1/m$)

The ratio of lateral strain to longitudinal (axial) strain within the elastic limit:

$$\nu = \left|\frac{\text{lateral strain}}{\text{longitudinal strain}}\right|$$

It is about 0.25 to 0.33 for most metals. The constants are related by $E = 2G(1+\nu) = 3K(1-2\nu)$.