to make someone or something else seem less important:
Her interest in politics began to overshadow her desire to be a poet.
The achievement of the men's team was overshadowed by the continuing success of the women's team.
to make an occasion or period of time less enjoyable by making people feel sad or worried:
The threat of war overshadowed the summer of 1939.
if a tall building, mountain etc overshadows a place, it is very close to it and much taller than it:
a dark valley overshadowed by towering peaks