NFA refers to Nondeterministic Finite Automaton. A Finite Automata(FA) is said to be non-deterministic if there is more than one possible transition from one state on the same input symbol. 
A non-deterministic finite automata is also a set of five tuples and represented as,

Where, 
Q: A set of non empty finite states. 
Σ: A set of non empty finite input symbols. 
δ: It is a transition function that takes a state from Q and an input symbol from and returns a subset of Q. 
qo: Initial state of NFA and member of Q. 
F: A non-empty set of final states and member of Q.