scar left on the twig after a leaf is shed.
Merriam-Webster Online Dictionary
leaf scar (noun) the mark left on a stem after a leaf falls
leaf scar (Wikipedia)
A leaf scar is the mark left by a leaf after it falls off the twig. It marks the site where the petiole attached to the stem. A leaf scar is typically found below a branch as branches come from axillary buds located above leaf scars.
The leaf scar on Ailanthus altissima