A nondeterministic 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 store an unbounded amount of information. The "nondeterministic" aspect means that the automaton can make multiple choices at each step, enabling it to explore many possible 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 state diagrams and transition functions, which define how the automaton moves between states based on input symbols and stack contents. This makes them a powerful tool in the field of formal language theory and compiler design.