What is giving a previous narrative or explanation, as of the plot or personages of a play; introductory. = 32,500 yrs.) Also called brevium, Uranium X2 and UX2 called?

protactic — Giving a previous narrative or explanation, as of the plot or personages of a play; introductory. = 32,500 yrs.) Also called brevium, Uranium X2 and UX2. (Webster's 1913.)