Question
I was wondering what exactly is a "site line"? I'm reading a lighting design article in a magazine and the phrase is constantly repeated, yet I'm not sure what it means. Could you explain?

Answer
When I was going to school, the instructors (rightly so) reinforced the point that certain terms have been over-used or poorly applied in the Landscape Architect's world.  I guess you'd call them "Professional Jargon".  

Some of them are:
Site vs. Sight
Charette (As in "Design Charette")
Design (What exactly does "design" or "art" mean? Better to be as specific as possible)
Node point
Bosc (which is a lovely Italian term for "grouping of trees", but mostly not understood by everybody else to mean a type of pear!)
Allees (A true sight line?
Rond points
Estuarine systems (Why not just say "waterway")
Fluvial morphology (Fancy way of saying "river")
Ecotones
Etc.

To your question - I think it is a miss understanding of one of the above terms.  Classic design calls for points of interest to be located within the landscape to be viewed from other points.  From geometry, of course, two points connect together with a line; the site's "sight" line, if you will. We also use sight lines at corner intersections to verify that there is not an unobstructed view of approaching cars, sometimes misspelled as a "site line".

Another big problem is poor use of grammar in planning and design documents. My two big ones are the over-use of the "passive voice" - By example, "It was discovered that the neighborhood has poor soil," versus a declarative sentence- "The neighborhood has poor soil."  The first sentence sounds impressive, but does soil really discover itself?

The other grammatical nut that "just gets to me" is the common practice of splitting the infinitive verb. The classic example is Star Trek's "To boldly go".  As any grammarian will tell you, it's "To go boldly".  I hope you're not a die-hard "trekie", Maya...

I know some of my answer is a bit off-topic, But I hope it helps.