perforation; tear-off line
Refers to a line of small holes made for easy tearing, like on paper forms, tickets, or packaging. The term comes from the resemblance to sewing-machine stitches.
この用紙にはミシン目が入っているので、簡単に切り離せます。
This paper has a perforation, so you can tear it off easily.
From ミシン (mishin, 'sewing machine') + 目 (me, 'line' or 'stitch'), because the perforations resemble a line of sewing-machine stitches.