- Where they are actually correct, such as dimensions (x, y, z) or vectors (j, k, l, u, v, w).
- Other mathematical symbols (N, M, e, t). Maths is also a special case in which I'm personally willing to do away with usual variable naming conventions, for example when translating formulas from a scientific paper into program code.
- Very short code, e.g. lambdas (lambda x: x2), list comprehensions ([p.title for p in posts]) or simple loops.
If you look at SQLite, you'll notice that most variable and struct field names are short and cryptic, and that library is still considered a high-quality piece of code.
Regardless, I don't understand all the downvotes you're getting.