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1.
Someone who always believes the items, fads, morals of the past are better than what is comparable today. Someone who fanatically believes that something today can never be as good as the thing of the past.
Guy 1 - "Man. These new games suck! They never can compare to the games on the Atari and Commodore 64!"

Guy 2 - "Dude, those games were neat, but quit being a nostalgist. What's past is past."
by Seth Rediker March 14, 2007
 
2.
(noun) One who looks constantly to nostalgia to save the day.
This word has been adopted by young culture in cities across the country. Regional slang synonyms include coconut boost and taleggio.
"Oh, I know!" exclaimed the nostalgist eagerly, "Remember Rosie and Jim?"
by Corkin July 31, 2006