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.
Between the time Eileen died and was 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 "God wound". 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.