Secret Ending refers to a secret video or cutscene that is unlocked after completing certain prerequisites in the Kingdom Hearts series and viewed after beating the main campaign in the games. These secret endings have been around since the first Kingdom Hearts and have continued to be a main staple of the series and have been included in almost every game.
Secret endings in the Kingdom Hearts series usually tend to be short cutscenes either rendered with pre-rendered CGI or rendered in-game that shows scenes or important plot points from future titles in the series.
Another side, Another story...
Another side, Another story... is the secret ending of Kingdom Hearts, and the first one of the series.
Another Side, Another Story [deep dive]
Another Side, Another Story [deep dive] is an expanded version of Another side, Another story... secret ending that appears in Kingdom Hearts Final Mix.
The Gathering is the secret ending of Kingdom Hearts II; the same scene is known as simply "Gathering" in Kingdom Hearts Birth by Sleep.
Birth by sleep
A fragmentary passage
Signs of What's Next
Signs of What's Next is the secret ending of Kingdom Hearts Re:coded.
Another Guardian of Light
Another Guardian of Light is the secret ending of Kingdom Hearts 3D: Dream Drop Distance.
Yozora is the secret ending of Kingdom Hearts III.