Single Letter Variables
Whether you use single letter variable names or longer ones depends entirely on context as well as what sort of domain you are working in. If you are writing some kind of sales software e.g. I doubt you would want to use many single letters.
However for a lot of scientific software that I work on, single letter names for a lot of concepts is already well established. It is common to name vectors things like v, u and w. For instance if you are implementing the dot product of two vectors it does not make sense to give them long names, that does not add extra information or value.