Just to repeat the above, almost any metal used in a bicycle (and perhaps other manufacturing) is an alloy of some sort. Iron, aluminum etc are apparently not suitable in their pure forms for bicycles.

Steel is by definition an alloy (usually of iron and carbon)... different steels add other materials (such as carbon, chromium etc)

If someone says alloy in reference to bikes, they do often mean aluminum... or more precisely aluminum alloy as opposed to steel alloy.