The layman term for drain cover (grate) is manhole cover. Say "manhole cover" and pictures 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 would prefer to call them) have been invented out of the necessity in order to avoid people from falling into underground sewage and also to prevent large objects from entering the drains which cause choking which eventually can cause flooding.
Fast forward to the modern day, the sewage system is still being employed under all cities around the world. As opposed to the crude stone manhole covers Jonite channel drain covers use in the ancient days, we now have special steel reinforced stone gratings that serve the thousand year old function of covering the inlets of our sewage system.
A lot more than a cover, check out Jonite's range of drain covers (drain gratings) here: http://www.jonite.us/products/creative-grates