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
Note: in print, a little extra information can be packed into the font used for a given character. The convention in Physics is that a character used as a symbol (like m for mass) is italicized. The letter m is also used (without italics) as an abbreviation for meter. Units are generally not italicized. Examples are shown in the second table below.