Nostalgic is a pleasurable yet sad feeling a person gets when reminiscing about things that happened in the past, and wishes it could be experienced again. Nostalgic can be used as an adjective or noun.
Johannes Hofer coined the word "nostalgia" in the 17th century as a disease suffered by Swiss soldiers who had a longing for a specific place or thing. Doctors treated nostalgia as a disease with solutions that ranged from the dubious cure of inflicting pain to a more sound cure of sending a patient home.
The adjective form of nostalgic means feeling both happy and sad when thinking about past experiences. For instance: I felt nostalgic memories after attending my high school reunion. According to Collins English dictionary, the noun form of nostalgic is "a person who indulges in nostalgia." For example: Leslie is a nostalgic, spending much of her time reminiscing about the past.