A Non-deterministic Pushdown Automaton (NPDA) is a theoretical model used in computer science to recognize context-free languages. It extends the capabilities of a finite automaton by incorporating a stack, which allows it to keep track of additional information. The "non-deterministic" aspect means that the automaton can have multiple possible transitions for a given state and input symbol, enabling it to explore different computational paths simultaneously. NPDAs are particularly useful for parsing nested structures, such as those found in programming languages and mathematical expressions. They can be represented using formal definitions, including states, input symbols, stack symbols, and transition functions. The study of NPDAs is essential in the fields of theoretical computer science and formal language theory.