The footprint of the building when viewed in plan resembles the shape of the letter X.
Note that this also includes crucifom-shaped plans (resembling the "+" sign).
A building with X-shaped plan, Vancouver, Canada (Map data ©2013 Google, Province of British Columbia, DigitalGlobe, IMTCAN)
A building with cruciform-shaped plan, Clock Tower Building, Kyoto University, Japan (left: Map data ©2013 Google, ZENRIN; right: C. Scawthorn)