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, a transition function, a start state, and a set of accept states. They can be converted into equivalent deterministic finite automatons (DFA) using algorithms like the subset construction method. NFAs are commonly used in applications such as lexical analysis and regular expression matching.