SB 3.11.36

tasyaiva cānte kalpo ’bhūd
 yaṁ pādmam abhicakṣate
yad dharer nābhi-sarasa
 āsīl loka-saroruham
tasya — of the Brāhma-kalpa; eva — certainly; ca — also; ante — at the end of; kalpaḥ — millennium; abhūt — came into existence; yam — which; pādmam — Pādma; abhicakṣate — is called; yat — in which; hareḥ — of the Personality of Godhead; nābhi — in the navel; sarasaḥ — from the reservoir of water; āsīt — there was; loka — of the universe; saroruham — lotus.
The millennium which followed the first Brāhma millennium is known as the Pādma-kalpa because in that millennium the universal lotus flower grew out of the navel reservoir of water of the Personality of Godhead, Hari.

The millennium following the Brāhma-kalpa is known as the Pādma-kalpa because the universal lotus grows in that millennium. The Pādma-kalpa is also called the Pitṛ-kalpa in certain Purāṇas.