In Jainism, “pakkhi” (also written pakhi / pakshik) usually refers to Pakshik Pratikraman.

Simple meaning:

• Pakkhi = fortnightly Pratikraman
• It is a special Pratikraman done once every lunar fortnight, so twice in a lunar month.
• Traditionally it is done on the 14th tithi (Chaturdashi) of each fortnight. ( jainknowledge.com)

Purpose:

• Same basic purpose as daily Pratikraman:

- to review faults of the last 15 days, - confess and repent, - ask forgiveness, - and take a new resolve not to repeat them.

• It is especially recommended for those who cannot do daily Pratikraman, as a minimum regular self‑purification. ( jainknowledge.com)

Both Śvetāmbara and Digambara traditions have the concept of Pakshik / Pakkhi Pratikraman; the internal sutras and method may differ, but the aim is the same – repentance and renewal over the past fortnight.

You can read a simple explanation of Pratikraman and its types here