Conversation so far:
what is m with a dot over it
The common m-dot symbol is used to notate the mass flow rate, the mass of a substance which passes per unit of time.
how is the mass flow rate defined
Mass flow rate is defined by the limit.
what fields use m-dot notation
The M-dot is used in physics and engineering.

Question: what can be used instead
Self-contained rewrite: what can be used instead of m-dot