The adjective limpid describes something (often liquid) that is clear, serene and bright. Nature calendars often feature glamour shots of a limpid stream or a limpid lake.
The adjective limpid may also describe language that is easily understandable. Your teacher might ask you to give an answer in a single limpid sentence. But he probably won't because limpid is a word that's fallen out of use. Maybe because it sounds too much like limp. Or maybe because it's associated with the king of all clichés: "Her eyes were like limpid pools."