1) to throw a shadow upon, to envelop in a shadow, to overshadow
From a vaporous cloud that casts a shadow the word is transferred to
a shining cloud surrounding and enveloping persons with brightness.
Used of the Holy Spirit exerting creative energy upon the womb of
the virgin Mary and impregnating it (a use of the word which seems
to have been drawn from the familiar OT idea of a cloud as
symbolising the immediate presence and power of God)