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 a deterministic finite automaton, an NFA can have multiple possible transitions for a given input symbol, allowing it 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, transition functions, a start state, and a set of accept states. They can be converted into equivalent deterministic finite automatons (DFAs), which have a single unique transition for each input symbol. NFAs are commonly used in applications like lexical analysis and regular expression matching.