In the vernacular we often use, 'the one that got away' typically denotes a person who was a significant part of our love life at some point, but for various reasons the relationship didn't work out or blossom into its full potential. It's a notion steeped in a mix of nostalgia, regret, and unresolved feelings like wish we could turn back the time to when everything was right and perfect.
The term signifies a missed opportunity in love. It's a sentiment beautifully captured in novels like 'The Great Gatsby', where Jay Gatsby is hopelessly enamored with his 'one that got away', Daisy.