The Collaborative International Dictionary of English v.0.48:

Water flag \Wa"ter flag`\ (Bot.)
   A European species of Iris (Iris Pseudacorus) having bright
   yellow flowers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Iris pseudacorus
    n 1: common yellow-flowered iris of Europe and North Africa,
         naturalized in United States and often cultivated [syn:
         yellow iris, yellow flag, yellow water flag, Iris
         pseudacorus]