While “ephemeral” has come to describe anything fleeting or not lasting, it comes from a Greek word that specifically means “lasting a day.” We now think of ephemeral things as lasting for a very brief period of time, not necessarily a day. There’s also a usage for “ephemeral” in biology — as an adjective, it can describe a plant with a short life cycle, and as a noun, it can be a name for the plants themselves.