Final answer:
The symbol '$' is used for the Push operation in the formal definition of a Pushdown Automaton (PDA).
Step-by-step explanation:
In the formal definition of a Pushdown Automaton (PDA), the symbol '$' is used for the Push operation. A PDA is a theoretical machine used in computer science and computational linguistics to model systems that have a stack-like memory. The symbol '$' indicates the operation of pushing a symbol onto the stack of the PDA.