What is optical density? I'm a zoology minor and we are doing protein estimation by colorimetric method. I have stumbled upon a term 'Optical density'. I don't understand the term well. Is it a measure of the extent of light that can pass through a particular object?
I've checked a related question of this community and it doesn't solve my question completely.
 A: For all intents and purposes, OD is the negative of the order of magnitude the factor by which the intensity of the light is reduced by the attenuating element with said OD.
In other words: OD = 6, means that the intensity will be reduced by a factor of 10 to the power of -6, a.k.a by a factor of a million.
A: You're a little confused  probably because there are two usages of the words "optical density". 
The first usage is as a synonym for refractive index, as described in the answers to the related question you cite. This is the commoner usage in physics.
The second usage is the total attenuation afforded by a protective screen, neutral density filter, laser goggles or the like. $ODx\; \lambda=y$ or even $ODx\; y$ means that the filter, goggles etc afford a power attenuation factor of $10^x$ at a light wavelength of $y$ or light wavelength range $y$. That is, the power transmitted through the filter is $10^{-x}$ of the incident power when the wavelength is as stated.
For example, laser goggles marked $OD7\;488{\rm nm}$ means that the goggles will reduce incident power at 488nm by a factor of $10^7$. Goggles marked with a lone wavelength rather than a wavelength range are always meant for use with a particular kind of laser. For example, the $OD7\;488{\rm nm}$ goggles are meant for use with an argon ion laser. You cannot rely on them using another source of wavelength 485nm, for example. 
For generic use, a wavelength range must be specified. So, for example, one often sees $OD7\;450{\rm nm} - 510{\rm nm}$, meaning, pretty obviously,  goggles that will give you seven orders of magnitude of attenuation over the whole range $450{\rm nm} \leq \lambda \leq 510{\rm nm}$. 
A: Optical density is a mathematical way of describing the extent of attenuation of visual lightwaves.
Density is the log of opacity.
Opacity is the reciprocal of transmittance.
So a transmittance of 50% of the light (half of the light) would have an opacity of 2.    The log of 2 = .3
So a density of .3 blocks half of the light energy.
A: The term Optical Density or ("OD") can have two different but related uses.

*

*The first is in relation to the refractive index.  I've included an image that will enable a quick grasp of this concept.




*The second use is in reference to to the following:


Optical density may also mean the absolute value of the logarithm with
base 10 of the power transmission factor of an optical attenuator:
For example, an optical density of 3 means that the optical power is
attenuated by the factor $10^3$ That would correspond to an
attenuation by 30 decibels.
If several attenuators are used in series, their optical densities can
simply be added.
To avoid ambiguity, it is better to use the term absorbance.

https://www.rp-photonics.com/optical_density.html
Author: Dr. Rüdiger Paschotta - https://www.rp-photonics.com/paschotta.html
To further the explanation above, I thought this image describing the second usage of optical density may be useful.  The concept is being applied to a lens in regards to a laser passing through it.  Each wavelength in the following chart will have its own OD but the notation is stating the minimum optical density over a range of wavelengths.

