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
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