Is definition #2 not exactly what you were saying the moral "convention" was for each individual society?
Perhaps. But conventions are not necessarily
popular, and they are often not arrived at by any matter of consensus or conscious deliberation. Your sarcastic remark about morals being handed down by gods is actually ironically sort of close to the mark- the existing morals in a given culture are usually values that have been
codified, most often times by a religion. Historically and from an evolutionary perspective, religion has been one of, if not
the, prime mechanism for enforcing and passing on norms and values, and regulating the behavior of members of a group. Now, its probably not the case that any of these prophets or religious leaders were actually given any ethical mandates from any deity (no Moses and the 10 commandments, in other words), but simply gave expression to existing societal behavioral patterns by documenting them in religious scriptures; encouraging altruism and cooperation (because this gives an evolutionary advantage to the members of the group acting altruistically), discouraging violence and cheating (since cheating detracts from the fitness of the overall group), and so on.
So even if #2 of the definition technically "fits", it is still slightly misleading- conventions are not like gallup polls, and in that sense they are not like opinions. Morals are more like other societal conventions and norms; washing your hands, dress codes, and so on. And in this sense they are exactly like the rules of chess; they aren't absolute rules, like "gravity is proportional to mass", but relative to a given societal moral game/set of conventions, it is true that "unnecessary violence is wrong/bad/immoral" just like it is true that "washing your hands is proper/right/etc."