What term is used for something that lasts for a very short time?

The term that signifies something lasting for a very short time is "ephemeral." This word is often used to describe phenomena that are fleeting and transient, such as certain flowers that bloom for only a day or experiences that are momentary. "Ephemeral" emphasizes brevity and the idea that the existence of something is short-lived, highlighting its temporary nature.

In contrast, "eternal" describes something that is everlasting or infinite, which directly opposes the concept of brevity. "Temporary" implies something that is not permanent but doesn't specifically convey the idea of a very short duration as strongly as "ephemeral." "Prolonged" refers to something that lasts for an extended period, which again is contrary to the idea of lasting only a brief time. Thus, "ephemeral" is the most accurate term for something that exists for a very short duration.