A "horizontal slit" refers to a narrow opening or cut that is positioned horizontally, meaning it goes from left to right or right to left rather than up and down. Full definition
Using a sharp knife, cut horizontal slits about halfway through roll, spacing every 1» along length. (epicurious.com)
JoJo, if you could add an extra horizontal slit higher up the next time round, it would be great! (jojomamanbebe.com)
It wears its roots with pride, boasting a tormented hero, femme fatale, and ever - present window blinds casting their ominous horizontal slits. (nextprojection.com)