The original colossus was an enormous statue that was supposed to have guarded the ancient Greek island and city of Rhodes. Now, though, we use the noun colossus for someone of huge importance, reputation, or influence.
In the world of American music, Louis Armstrong is a colossus. He invented or perfected many of the elements of what we now call jazz — there's probably no figure of greater importance in that field. Even the second most important athlete in the world seems small in comparison to the colossus that is Muhammad Ali. He is, quite simply, the greatest. In the ancient world, Rome was an imperial colossus. Few nations could rival its strength, size, or colossal growth.