A Pushdown Automaton (PDA) is a type of computational model that extends the capabilities of Finite Automata by incorporating a stack as part of its computation mechanism. This enhancement allows PDAs to recognize a broader class of languages, specifically context-free languages, which cannot be fully captured by Finite Automata.
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A finite control equipped with an unbounded stack. A move reads one input symbol or the empty word, examines the top stack symbol, changes control state and replaces that top by a finite word over the stack alphabet. Nondeterministic choices give finitely many possible moves. With acceptance by final state and the entire input consumed, pushdown automata recognize exactly the context-free languages.