What is the physical meaning of Poisson's ratio?

What is the physical meaning of Poisson's ratio?

In the range of elasticity, most materials obey Hooke's law, that is, the deformation is directly proportional to the force. The proportional constant of longitudinal stress and longitudinal strain is the elastic modulus E, also known as young's modulus
The ratio of transverse strain to longitudinal strain is called Poisson's ratio μ, which is also called transverse deformation coefficient