Flanking route
Also said as Flank, Side route, Third lane
A second way into a fight that comes at the enemy from the side or behind. It exists so that a defender can never watch everything at once.
It breaks stalemates: strong positions stop being unbeatable, and they need watching. It also makes you look over your shoulder, which is much of what makes a shooter tense.
Say it
“The hill is too strong: nothing reaches it except up the front. Add a side tunnel that comes out behind the top, longer by about ten seconds of running.”
Where you’ve seen it
Dust II in Counter-Strike is three lanes joined at the middle: a team holding one bombsite can be hit from mid as well as from the long route, so you can't just stare down one lane.
Not to be confused with shortcut
A shortcut saves time for someone going the same way. A flanking route may take longer, because what it buys is the angle you arrive from.