Password-Linked

Although the term Password-Linked is never used in-game, it is commonly used by fans and strategy guides to describe situations in Oracle of Ages or Oracle of Seasons. If something is "password-linked" in Oracle of Seasons, it can usually only be accessed with a password that can only be obtained by completing Ages, and vice versa.

There are situations in which passwords are given in either game before they are completed. These passwords will often grant the player to do something unique in the corresponding game, in which case the term "Password-Linked" is also often applied, although the term seems to be used more loosely in these predicaments.

The secrets are recorded in Farore's Memory in the Hall of Secrets.

Password-linked series ending
In this theory, the Adult Timeline ends with the ALttP/LA arc and the Child Timeline ends with the NES arc. Twinrova, who never died in the Child Timeline, returned to Hyrule using the power to cross freely between Termina and Hyrule. During Link's childhood in OoT, Twinrova pulls Dark Link, who in this theory is Link's Terminian counterpart, out of Termina and into Hyrule, and made Dark Link incapable of dying of old age. Since Twinrova and Dark Link did not die during the child timeline, after the events of OoT in the child timeline they went to the Room of Rites and resurrected Ganon in LoZ (although to light the Flame of Despair, instead of sacrificing Zelda they just shattered the Triforce of Wisdom.) After Ganon and Dark Link were slain in the 8-bit arc, and after Ganon had also been slain in ALttP, both timelines, having been split, were reunited by the Golden Goddesses, who took human forms in OoX and created a merged timeline from the adult timeline, with the merged timeline including New Hyrule from the adult timeline, and Holodrum and Labrynna, two new worlds connected through time to the child and adult timelines respectively. Leading up to OoS and OoA, Twinrova discovered the Merged Timeline and came there using the magic that allows them to cross between Hyrule and Termina freely to build a Room of Rites in New Hyrule to resurrect Ganon. The Golden Goddesses discovered this, but at the time they were not able to return to their goddess forms, having used a great deal of their power to create these new worlds, so the last Link of each timeline, child and adult respectively, were brought to Holodrum and Labrynna and in the process, fused into Merged Link, sent to the Merged Timeline to stop Twinrova. The password-linking fully fused Merged Link, and it gave him the power to fight Twinrova and the semi-revived Ganon, thus freeing the world from Ganon, effectively ending the Zelda series in the Merged Timeline.