The phrase "outer third" refers to the area or section that is situated towards the outside edge of something, divided into three equal parts. Full definition
Blade Dynamics have created a new and highly innovative «Advanced Blade Tip», which improves the critical outer third of a wind turbine blade. (windsystemsmag.com)
Hair loss — a sign that your thyroid is off, particularly from the head, eyelashes, and outer third of the brows (saragottfriedmd.com)
Take one outer third, either the right or the left, and fold it in toward the middle, then fold the other outer third in toward the middle, on top of the first folded third. (theredheadbaker.com)