The phrase "center seam" refers to a straight line or stitching found in the middle of an object or piece of clothing. It is a descriptive term used to indicate that the object has been divided into two equal halves. Full definition
We kept the body modern and architectural on this swinging tee with a clean center seam front and back, and used French seams on the inside. (garnethill.com)
Cut off the entire center seam, cutting up one leg and down the other. (seamstresserin.com)
You can see in the photo below that we were able to find two with similar center seams, and I actually liked it! (cottonstem.com)