tasyaiva cante kalpo ’bhud
yam padmam abhicaksate
yad dharer nabhi-sarasa
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The millennium which followed the first Brahma millennium is known as the Padma-kalpa because in that millennium the universal lotus flower grew out of the navel reservoir of water of the Personality of Godhead, Hari.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The millennium following the Brahma-kalpa is known as the Padma-kalpa because the universal lotus grows in that millennium. The Padma-kalpa is also called the Pitr-kalpa in certain Puranas.