Joshua tree (noun) a yucca that grows as a tree and has clusters of spiky leaves, native to arid regions of southwestern North America.
• Yucca brevifolia, family Agavaceae.ORIGIN mid 19th cent.: apparently from Joshua (Josh. 8:18), the plant being likened to a man brandishing a spear. (?)
P.S This is the actual tree that U2 used for thier album jacket photo to their "Yoshua Tree" platinum album.