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 (DFA), an NFA can have multiple possible transitions for a given input symbol, including transitions to multiple states or even no transition at all. This means that for some inputs, the NFA can be in several states simultaneously. NFAs are defined by a set of states, an input alphabet, a transition function, a start state, and a set of accept states. They are particularly useful in regular expressions and lexical analysis, where they can efficiently represent complex patterns. Although NFAs can be more intuitive, they can be converted into equivalent DFAs for practical implementation.