The Collaborative International Dictionary of English v.0.48:

Overshadow \O`ver*shad"ow\, v. t. [imp. & p. p. Overshadowed;
   p. pr. & vb. n. Overshadowing. ] [Cf. Overshade. ]
   1. To throw a shadow, or shade, over; to darken; to obscure.
      [1913 Webster]

            There was a cloud that overshadowed them. --Mark ix.
                                                  7.
      [1913 Webster]

   2. Fig.: To cover with a superior influence; to be viewed as
      more important than. --Milton.
      [1913 Webster +PJC]

   3. To cause to be sad or disappointing; to cast a sad shadow
      on; as, an accidental death overshadowed the joy of the
      festival.
      [PJC]

WordNet (r) 3.0 (2006):

overshadow
    v 1: be greater in significance than; "the tragedy overshadowed
         the couple's happiness" [syn: overshadow, dominate,
         eclipse]
    2: make appear small by comparison; "This year's debt dwarfs
       that of last year" [syn: shadow, overshadow, dwarf]
    3: cast a shadow upon; "The tall tree overshadowed the house"