"Cons" is an informal term referring to the opposition or negative side of a debate or to those points not in favour of the central motion. In other words, the "Cons" of a topic negate and deny the motion. "Cons" may also refer to the disadvantages of a plan or the negative perspective of an issue. The opposite of "Cons" is "Pros."
While its counterpart, "Pro" is sometimes used to indicate that one is in favour of a motion (for example, "I am pro or for the motion," as a variation of "I am in favour of the motion," or "I support the motion"), "Con" is not used in that way. Rather, to indicate a "Con" position, one would say, "I am not for the motion," "I am against the motion," "I am not in favour of the motion," or "I do not support the motion."