Sentence:  QMA arises naturally in the study of quantum computation, and it also has a <WSD>complete</WSD> problem, Local Hamiltonian, which is a generalization of "k"-SAT. 
Meaning of the word marked with <WSD> tags:  That is in a given complexity class and is such that every other problem in the class can be reduced to it (usually in polynomial time or logarithmic space).