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, NFAs can have multiple possible transitions for a given state and input symbol, allowing them to explore many paths simultaneously. This means that an NFA can be in several states at once, making it more flexible in recognizing certain languages. NFAs are defined by a set of states, an input alphabet, transition functions, a start state, and one or more accept states. They are particularly useful in regular expressions and lexical analysis, where they can efficiently match patterns in text. Despite their complexity, NFAs can be converted into equivalent deterministic finite automata (DFA) for practical implementation.