What is optical density?

Optical density is a term used in the field of optical spectroscopy to describe the propagation of a wave through a material. Optical density is also referred to as the absorbance of the medium.
The optical density of the medium influences the speed of light to a great extent. The speed of electromagnetic waves in a material with higher optical density will be slow.The optical density is not related with the density of the medium.The refractive index of the material is an indicator of its optical density.