Old Flame

A couple who have drifted apart for various reasons (not necessary Star-Crossed Lovers), only to discover they still have feelings for each other when they get together a lot later. The key part of this trope is that the feelings (even if one-sided) persist after their reunion.

Subtropes include New Old Flame (the old flame minus any sort of foreshadowing) and Old Flame Fizzle (the old flame minus the "flame" part). Contrast Working with the Ex, wherein no romantic feelings remain long before the reunion.

Examples of Old Flame include:

Comic Books

Scrooge McDuck and Goldie O'Gilt from the Disney Ducks Comic Universe. Between Scrooge leaving the Klondike and their reunion in the Carl Barks story Back to the Klondike they haven't seen each other for decades! (According to Don Rosa, Scrooge was circa 32 at the end of his Klondike years, and older than 80 years in stories taking place in the "present".)

Film

In RED (film), Victoria and Ivan are each other's old flames. They were assassins on opposite sides of the Cold War, and she was assigned to kill him. He wears the scar in his chest proudly because he knows if she hadn't loved him, she would've shot him in the head instead. He gets to get her out of a jam, and treats it like a great romantic moment.

Literature

Elaine and Harry from The Dresden Files. Harry, at least, still feels something for her.
Lampshaded in an Erast Fandorin short story Tea in Bristol, wherein the victim's old flame actually goes by the surname "Flame" (and is well in her fifties).
In Agatha Christie's The Hollow, John Angkatell leaves both his wife and his mistress for an old girlfriend who dumped him because he wouldn't sacrifice his career for her. As a result, he gets murdered. By his possessively-loving wife.
The Helmsman Saga has Margot Effer'wyck, Wilf Brim's love, who seems to take this Up to Eleven. She got married to other men twice (and had a kid), was presumed dead, had a drug addiction which forced her to betray Wilf, he finds other women every book and believed more than once that he truly left her and his feelings behind... Somehow, she always manages to pop back. The (currently) last book leaves an open ending on the matter.

Live Action TV

Highlander did one in 'Glory Days', Joe was worried his high school flame that stopped in to visit would think him damaged due to losing his legs, but once she found out, it seemed for a bit that things were okay-until she confessed she was married and couldn't stay.
CSI is hinting around at this since the arrival of Julie Finn. It's known that D.B. never cheated on his wife, thanks to Word of God, but it's been implied they had something before D.B. met his wife. Possibly another high school scenario, since D.B. and his wife met there too.
As Time Goes By is about a pair who were lovers as youth; then they meet again after both have lost their respective spouses, and renew their romance as seniors.

Video Games

The Yuktobanian recon Major Nastasya Obertas for Captain Bartlett in Ace Combat 5 The Unsung War.