Sam/Eileen fanworks pair the characters Sam Winchester and Eileen Leahy in a romantic or sexual relationship. The pairing is sometimes referred to by the portmanteau Saileen.
Fans spotted a connection between Sam and Eileen when they first meet while on a banshee hunt in 11.11 Into the Mystic.
In 12.17 The British Invasion, they share a hug.
Between the time Eileen died and is revived, Sam has been practicing sign language for the first time since college. His improved use of sign language is seen in 15.06 Golden Time and 15.07 Last Call.
In 15.07 Last Call, following her resurrection, Sam and Eileen spend more time together, including a night of many margaritas. As Dean leaves to go on a hunt, he teases Sam about his burgeoning relationship with Eileen, telling a protesting Sam to leave a sock on the door if needed. During a session of research (not a euphemism), they make moves towards a possible kiss when they are interrupted by Castiel's return.
In 15.09 The Trap Sam and Eileen find out her resurrection was orchestrated by Chuck, who makes her dig into Sam's wound from shooting Chuck with the Equalizer. After Chuck disappears, Eileen decides to leave despite Sam's protestations, saying that she doesn't know what is real anymore. Sam and Eileen kiss and Sam says "I know this is real", and then Eileen leaves.
In 15.14 Last Holiday, Sam goes on a date with Eileen. He dresses up and Mrs. Butters even gives him a bouquet of flowers to give her. Sam returns to the Bunker very late, missing his tie, but he doesn't reveal what happened during their date, despite Dean's teasing.
In 15.18 Despair, friends of the boys are disappearing unexpectedly, and Sam expresses worry (again; he also expressed this same worry in 15.17 Unity when Billie's plan is first revealed) that Eileen could be one of the victims. He drives (with Dean, Castiel, and Jack) to her house, texting her on the way that she should wait at her car for them to arrive. When they get there, only her keys, purse, and broken cell phone remain. Her phone's lock screen is Sam in the Bunker, smiling at the camera. When Dean expresses sympathy that Eileen is gone, Sam replies of his loss, "No, I can't... If I let myself go, then I'll lose my mind. I can't right now."
In 15.19 Inherit the Earth, after Jack steals Chuck's powers and becomes the new God, he restores all life on Earth, including Eileen. In 15.20 Carry On, at some point in the future, Sam is seen married to an unknown woman, though it's unclear what happened to Eileen by that time.