The baseball Benjamin Sisko kept on his desk aboard Deep Space Nine, which became a signal of his intention to return whenever he was forced to leave the station.
Sisko loves an Earth sport that hardly anyone has played for centuries, and a baseball sits on a stand in his office from the first season. He uses the game to explain linear time to the Prophets in the pilot, Emissary.
The ball gains its meaning in Call to Arms. When Starfleet evacuates the station ahead of the Dominion, Sisko leaves it on the desk, and Dukat understands at once that it is a message that he means to come back. Dukat keeps it beside him throughout the occupation and hands it over, broken in mind, after Sacrifice of Angels.
In Tears of the Prophets, a grieving Sisko goes back to Earth and takes the baseball with him, which tells Kira that this time he may not return. He brings it home at the start of the final season.