How about g_
(g underscore)
A quick look will allow you to realize that it's a global variable.
Recently, I spent some time programming in c#. No global variables allowed at all. Sometimes, they make things so easy.
Unless they're being shared by multiple threads, in which case you have to be sure to use a locking construct (mutex)
Microcontrollers are a different animal. You're not running more than one thread, so there's no risk.
But you have to be careful if the same global variable is being accessed from more than one interrupt.
(Break out the atomic locking gizmo...
)