line of sight

from The American Heritage® Dictionary of the English Language, 4th Edition

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

from Wiktionary, Creative Commons Attribution/Share-Alike License

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

from the GNU version of the Collaborative International Dictionary of English

• n. 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.

• n. an imaginary straight line along which an observer looks