The layman term for drain cover (grate) is manhole cover. Say "manhole cover" and Jonite Stone Gratings of spies/heros using 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 prefer to call them) have already been invented out of the basic need in order to avoid people from falling into underground sewage as well as to prevent large objects from entering the drains which cause choking which eventually may lead to flooding.
Fast forward to the current day, the sewage system is still in use under all cities all over the world. As opposed to crude stone manhole covers the Romans use in the ancient days, nowadays we have special steel reinforced stone gratings that provide the thousand year old function of covering the inlets of our sewage system.
More than merely a cover, check out Jonite's range of drain covers (drain gratings) here: http://www.jonite.us/products/creative-grates