Does the usage of mathematical symbols work differently in books than in theses? From what I know, one cannot use a mathematical symbol for two different notations in a thesis. Is this right? The thing is that I am trying to cite a book in my thesis, but this book uses (tau) for both viscous stresses and reynolds stresses. Can I do the same in my thesis or do I have to choose another notation?

Bear in mind that I am new to academic writing, thanks!
So why not just define a tau with a subscript v and a tau with a subscript r - make it clear in your nomenclature and you're sorted.

See: