Prismatic Dress (also known as Prism Dress in the SNES/PS version) is an armor in Chrono Trigger. Upon completion of the The Rainbow Shell sidequest, the Rainbow Shell can be forged by Melchior into either three Prism Helm or one Prism Dress. If you have the Sun Stone, then he'll make some PrismSpecs (an accessory that greatly boosts damage).
The Prismatic Dress can also be charmed by Ayla from the right hand of Queen Zeal. It can only be worn by Ayla, Lucca, and Marle.
Up to four Prismatic Dresses can be obtained per playthrough if the player completes the Black Omen dungeon in the Present, the Past, and finally Antiquity. The order is important: destroying the Omen causes it to disappear in later time periods as well. This cannot be done in the DS Remake as charming Queen Zeal for the dress now applies to all timelines.