A Sight

1. (in the singular) The ability to see.
2. The act of seeing; perception of objects by the eye; view.
3. Something seen.
4. Something worth seeing; a spectacle, either good or bad.
5. A device used in aiming a projectile, through which the person aiming looks at the intended target.
6. A small aperture through which objects are to be seen, and by which their direction is settled or ascertained.
7. (now colloquial) a great deal, a lot; frequently used to intensify a comparative.
8. In a drawing, picture, etc., that part of the surface, as of paper or canvas, which is within the frame or the border or margin. In a frame, the open space, the opening.
9. (obsolete) The instrument of seeing; the eye.
10. Mental view; opinion; judgment.

Synonyms

• (ability to see): sense of sight, vision
• (something seen): view
• (aiming device): scope, peep sight

Astrophysics:

line of sight:

in Doppler Effect is the line connecting the source and observer.