The phrase "armhole seam" refers to the sewing line that connects the body of a garment with its sleeve, where the arm goes. It is the seam that runs along the underarm area. Full definition
It's what I have to do, but only a wee bit, just by increasing the length of the back armhole seam by 1/2 inch and shortening the front by 1/2 inch but leaving the neck as is. (acolourfulcanvas.com)
By lengthening the back armhole seam and shortening the front.... (acolourfulcanvas.com)
For the most natural shoulder / sleeve silhouette, press the armhole seam allowances open about 4 in. (threadsmagazine.com)