line of sight

noun
1.
Also called line of sighting. an imaginary straight line running through the aligned sights of a firearm, surveying equipment, etc.
2.
Astronomy. an imaginary line from an observer to a celestial body, coincident with the path traveled by light rays receivedfrom the body.
3.
Radio. a straight line connecting two points sufficiently high and near one another so that the line is entirely above the surface of the earth.
4.
Ophthalmology, line of vision.

Origin:
1550–60

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
line of sight
 
n
1.  the straight line along which an observer looks or a beam of radiation travels
2.  ophthalmol another term for line of vision

Collins English Dictionary - Complete & Unabridged 10th Edition
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Example sentences
Conventional microscopes use lenses to magnify whatever is in the line of sight.
Lasers provide a better long-distance solution-but only if there is direct line
  of sight between source and device.
But that only allows correction for distortions along one line of sight.
As you can see, it's tilted pretty severely to our line of sight.
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