GNU Collaborative International Dictionary of English

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1.       
   Macron (?), n. [NL., fr. Gr.  long.] (Pron.) A short, straight, horizontal mark [-], placed over vowels to denote that they are to be pronounced with a long sound; as, ā, in dāme; ē, in sēam, etc.
   
   Syn. -- macrotone.
   
   [1913 Webster]