This document discusses nondeterministic finite automata (NFAs) as a relaxation of deterministic finite automata (DFAs). NFAs allow multiple possible transitions from a state on a given input, whereas DFAs require exactly one transition. An NFA is defined as a 5-tuple with a transition function that maps a state-input pair to a set of possible next states. A string is accepted by an NFA if there exists at least one sequence of transitions leading from the start state to an accepting state on that string.