line of sight

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An imaginary line from the eye to a perceived object.
• noun An unobstructed path between sending and receiving antennas.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A straight line along which an observer has a clear view.
• noun weaponry The line which passes through the front and rear sight, at any elevation, when they are sighted at an object.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an imaginary straight line along which an observer looks