Definition of daylighting:
(N) :
the practice of placing windows, skylights, other openings, and reflective surfaces so that direct or indirect sunlight can provide effective internal lighting.
(N) :
the opening up and restoration of a previously buried watercourse, one which had at some point been diverted below ground.
(N) :
Daylighting a tunnel is to remove its "roof" of overlying rock and soil, exposing the railway or roadway to daylight and converting it to a railway or roadway cut.