xD-Picture Card is the proper form of the name—not XD- Use of the uppercase X is due to a known limitation of the MediaWiki software used for this site.
Olympus 2 GB xD-Picture Cardimage by Huang Yu Ming (Image rights)
Like Sony's Memory Stick, xD was not as successful as hoped and fell technically behind Secure Digital. For this reason, Fuji began double xD / Secure Digital slots in their compact cameras from the FinePix F40fd onward. Olympus was more resistant to dropping the xD format, instead adding software rather than physical compatibility for SD, allowing smaller form factor SD cards to be used via an adaptor. Around 2009 Olympus phased out xD in favor of Secure Digital/SDHC cards alone.