Webster's English Dictionary

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estuary

es.tu.ary \'es(h)-ch*-.wer-e-\ n [L aestuarium, fr. aestus boiling, tide; 
   akin to L aestas sum] often attrib mer - more at AESTIVAL : a water passage 
   where the tide meets a river current; esp : an a rm of the sea at the lower 
   end of a river