A formal system is comprised of axioms and inference rules. Each axiom and inference rule is defined syntactically, that is, by ordered sequences of symbols that follow strict syntactic rules but do not (necessarily) have any meaning. The set of all symbols allowed in a formal system are explicitly listed in its alphabet, simply, a finite, non-empty set of symbols.
My formal system, let's call it FS, uses the alphabet {E,0,1} and has only one axiom and two inference rules. Here is the axiom (in the box below):
