What is OD260?

OD is a physical property of solutions defined as "Optical Density".
It can be described as "how much light is absorbed by a solution at a given concentration when the solution is is put in a cuvette of a given width."
The width of a cuvette is determining how many millimeters will the beam of light travel through the solution.
The higher the concentration of a solution - the higher is the optical density of the solution.

So, how to formularize the things said above?
Beer and Lambert derived an equation that is known as Beer-Lambert Law (or sometimes just Beer's Law).
Extinction coefficient is written as a small Greek letter epsilon () or just e.
Absorbance is written as A, concentration as c and length of the cuvette (in cm) as l.
The formula for this law is:

A = e x c x l