The opposite of the term "un prolonged" is "prolonged," which means something that is stretched out for a long period. Other antonyms of this term are "extended," "protracted," and "lengthy," which all imply a situation or activity that lasts longer than normal. Other antonyms of "un prolonged" are "brief," "short-lived," and "transient," which imply a situation or activity that lasts for a short period. These terms all convey the sense of something that is short, concise, and quickly finished. Choosing the right antonym to use depends on the context and the message the writer wants to convey.