In Physics we like to use a very compact notation for things we talk about a lot; this is æsthetically mandated by our commitment to making complicated things look [and maybe even be] simpler. Ideally we would like to have a single character to represent each paradigmatic ``thing'' in our lexicon, but in practice we don't have enough characters3.5 and we have to re-use some of them in different contexts, just like English!
In principle, any symbol can be used to represent any quantity, or even a non-quantity (like an ``operator''), as long as it is explicitly and carefully defined. In practice, life is easier with some ``default'' conventions for what various symbols should be assumed to mean unless otherwise specified. On the next pages are some that I will be using a lot.3.6