A nondeterministic finite automaton (NFA) is a theoretical model used in computer science to represent and recognize patterns within input strings. Unlike deterministic finite automata (DFAs), NFAs can have multiple possible transitions for a given state and input symbol, allowing them to explore many paths simultaneously. This flexibility makes NFAs useful for certain types of computations and pattern matching. In an NFA, a string is accepted if there exists at least one path through the automaton that leads to an accepting state. NFAs can be converted into equivalent DFAs, which can simplify the implementation of algorithms for tasks like lexical analysis in compilers.