(1) Lasting a day only. (2) Lasting a very short time.
Example:
The benefits from the strategy will only be ephemeral, but we'll be paying for it for years to come.
Something that is literally ephemeral is "over" in a day, hemera being the Greek word for "day." Ephemeral plants such as day-lilies have blooms that last only a day. More often, though, ephemeral is not to be taken quite so literally. In the world of show business, for example, fame is apt to be breathtakingly ephemeral, a year in the limelight followed by total obscurity.