The layman term for drain cover (grate) is manhole cover. Say "manhole cover" and images of spies/heros making shower channel drain of the underground sewage system popping out of manhole cover in Hollywood films appear.
Historically from the time that Man invented the sewage system since the Roman Times to drain water and urban waste away for sanitation purposes, manhole covers (drain covers or grates as we would prefer to call them) have already been invented out from the basic need in order to avoid people from falling into underground sewage and to prevent large objects from entering the drains which cause choking which eventually may lead to flooding.
Fast forward to the present day, the sewage system is still being employed under all cities worldwide. As opposed to crude stone manhole covers the Romans use in the ancient days, we now have special steel reinforced stone gratings that provide the thousand year old function of covering the inlets of our sewage system.
Far more than a cover, check out Jonite's range of drain covers (drain gratings) here: http://www.jonite.us/products/creative-grates