– pleasure and sadness that is caused by remembering something from the past and wishing that you could experience it again

Source: Merriam-Webster Dictionary

This word came up in a class discussion we had. I had a faint idea of what it meant, but after looking it up, I came to understand that it was associated with more than just “remembering”  but rather also with a feeling of downhearted melancholy with a certain memory. Originally I just assumed it was just a positive feeling of thinking of a memory.