Source : Webster's Revised Unabridged Dictionary (1913)

Adonai \Ad`o*na"i\, n. [Heb. ad[=o]n[=a]i, lit., my lord.]
   A Hebrew name for God, usually translated in the Old
   Testament by the word ``Lord''.

   Note: The later Jews used its vowel points to fill out the
         tetragrammaton {Yhvh}, or {Ihvh}, ``the incommunicable
         name,'' and in reading substituted ``Adonai''.