The layman term for drain cover (grate) is manhole cover. Say "manhole cover" and pictures of spies/heros making use of the underground sewage system popping out of manhole cover in Hollywood films appear.
Historically ever since 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 necessity to avoid people from falling into underground sewage and to prevent large objects from entering the drains which cause choking which eventually can result in flooding.
Fast forward to the current day, the sewage system is still utilised under all cities around the world. As opposed to crude stone manhole covers the Romans used in the ancient days, now we have special steel reinforced stone gratings that serve the thousand year old function of covering the inlets of our sewage system.
Much more than just a cover, check out Jonite's range of drain covers (drain gratings) here: http://www.jonite.us/products/creative-grates