A nondeterministic finite automaton (NFA) is a theoretical model used in computer science to represent and analyze the behavior of certain types of computational systems. Unlike deterministic finite automata (DFAs), NFAs can have multiple possible transitions for a given input symbol, allowing them to be in several states at once. This flexibility makes NFAs useful for recognizing patterns in strings and languages. NFAs are defined by a set of states, an input alphabet, a transition function, a start state, and a set of accept states. They can process input strings by exploring all possible state transitions simultaneously, which can simplify the design of algorithms for pattern matching and lexical analysis in programming languages, such as those used in compilers.