"Is Consistency is a semantic or a syntactic quality ?"
In first-order logic, as the other answers indicate, the completeness theorem shows that syntactic and semantic consistency are equivalent.
In other logics, the two types of consistency are distinct, and you have to specify the type of consistency you mean. In particular, to give a very specific example, there is no complete effective, sound deductive system for second-order logic, and given any particular effective, sound deduction system for it there are effective theories that are syntactically consistent but semantically inconsistent.
Things can get even more odd as we go farther from classical logic. Classically, it is equivalent to say that a theory proves $\phi \land \lnot \phi$ for some $\phi$, and to say that the theory proves every sentence $\phi$. But in other logics these may not be equivalent, so they may represent distinct types of syntactic inconsistency. Paraconsistent logics, in particular, allow for some contradictory statements to be proved without allowing every statement to be proved.