[1] : The Collaborative International Dictionary of English v.0.48

midnight sun \mid"night` sun\
   the sun shining at midnight in the arctic or antarctic
   summer.
   webster 1913 suppl.

[2] : WordNet (r) 2.0

midnight sun
     n : the sun visible at midnight inside the arctic or antarctic
         circles

Midnight Sun >> M Words
Midnight Sun, definition of term: Midnight Sun
midnight+sun_pag1.html

Midnight >> M Words
Midnight, definition of term: Midnight
midnight_pag1.html

Sun >> S Words
Sun, definition of term: Sun
sun_pag1.html

Sun Spots >> S Words
Sun Spots, definition of term: Sun Spots
sun+spots_pag1.html

Sun God >> S Words
Sun God, definition of term: Sun God
sun+god_pag1.html

Sun Bear >> S Words
Sun Bear, definition of term: Sun Bear
sun+bear_pag1.html