In computational complexity theory, QMA, which stands for Quantum Merlin Arthur, is the quantum analog of the nonprobabilistic complexity class NP or the probabilistic complexity class MA. It is related to BQP in the same way NP is related to P, or MA is related to BPP.
Informally, it is the set of decision problems for which when the answer is YES, there is a polynomial-size quantum proof (a quantum state) which convinces a polynomial-time quantum verifier of the fact with high probability. Moreover, when the answer is NO, every polynomial-size quantum state is rejected by the verifier with high probability.
More precisely, the proofs have to be verifiable in polynomial time on a quantum computer, such that if the answer is indeed YES, the verifier accepts a correct proof with probability greater than 2/3, and if the answer is NO, then there is no proof which convinces the verifier to accept with probability greater than 1/3. As is usually the case, the constants 2/3 and 1/3 can be changed. Changing 2/3 to any constant strictly between 1/2 and 1, or changing 1/3 to any constant strictly between 0 and 1/2, does not change the class QMA.
QAM is a related complexity class, in which fictional agents Arthur and Merlin carry out the sequence: Arthur generates a random string, Merlin answers with a quantum certificate and Arthur verifies it as a BQP machine.