Something you had once, or even regulalry, in the past, that you can't get now.
For instance when I was in my mid 20s there was this Italian bakery where I lived. Two things I used to get there. Their homemade apple turnovers, and their fried egg sandwiches on a poppy seed hard roll. Of course that was 30 years ago, but I still know exactly how both those things tasted and I long for them! For 7 years I enjoyed those things.
No other turnover or egg sandwich has ever come close.
Wolf Brand chili. This is a food from my early 20s, when I lived in Houston Tx. When I first came back to the northeast I was so sad I couldn't find it here. I could get it on Amazon now, but there are two things stopping me 1) I doubt after all these years it's the same recipe, and it probably taste like crap now, and 2) I don't want to be stuck with a case of it, if it's awful.
After I lost the Wolf Brand I turned to Campbell's Fiesta chili beef soup. But they did away with that a long time ago now. Beside, Campbell's is awful now. That's why I learned to make my own soup. I havne't eaten Campbell's in years.
But mostly it's those turnovers and fried egg sandwiches.