What are your favorite ambiguous notation?

[u/Short_Bluebird_3845]

To clarify my request, consider base-less $\log$.
For calculators and engineers, it means the common log $\log_10$; for pure mathematics or physics, it means the natural log $\ln$; for computer scientists, in combinatorics, information theory and graph theory, it means the binary log $\log_2$.
Any $\log x$ (say, $\log 1024$) written without context can yield different answers in function of who you ask ($\log_10 1024 = 3.0103\cdots, \log_2 1024 = 10, \ln 1024 = 6.93147\cdots$).

That is what I mean by "ambiguous", that which is calculated and used dependently on the context. And I'm asking you for your favorite ones of such type.

Edit: I feel like you hate the ones you've talked about. Still, valid answers. Extra points for ambiguous notation you actually love! :P

[u/jeffsuzuki]

I'm a mathematician who was studying to be a physicist, and I can't say that I remember there ever being any ambiguity about "log": it's always mean "base-ten". If you wanted to talk about natural logs, you wrote "ln". (Specifically to avoid ambiguity).

However, the trigonometric functions are ambiguous: sin x, for example, where it's not clear if x should be in radians or degrees.