The Collaborative International Dictionary of English v.0.48:

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".
   [1913 Webster]

   Note: The later Jews used its vowel points to fill out the
         tetragrammaton Yhvh, or Ihvh, "the incommunicable
         name," and in reading substituted "Adonai".
         [Webster 1913 Suppl.]