Source : Webster's Revised Unabridged Dictionary (1913)

Water flag \Wa"ter flag`\ (Bot.)
   A European species of Iris ({Iris Pseudacorus}) having bright
   yellow flowers.

Source : WordNet®

Iris pseudacorus
     n : common yellow-flowered iris of Europe and North Africa,
         naturalized in United States and often cultivated [syn: {yellow
         iris}, {yellow flag}, {yellow water flag}]