A remembrance of something that was important to you, something that you loved, something you heard as a infant or a toddler, something that is unforgettable.
Me growing up hearing the Final Fantasy VII soundtrack, now thats nostalgia to me.
When your nostalgia makes you act stupid. Usually hate whatever is new and prefer the old version just because of nostalgia.
Person 1: "Dude, the original GTA was the bomb. GTA V sucks"
Person 2: "You're nostalgia biased"
Person 1: "Omg, Dragon Ball Z was so much better than Super!"
Person 2: "Reason?"
Person 1: "Because I watched it when I was a kid! Plus it's just better"
Person 2: "Nostalgia Bias"
The longing for or reminiscence of an era which you did not actually grow up in, having only experienced the era through it's movies, TV shows, music, etc; not based on actual life experiences.
Most people these days claiming to be 90s kids are full of false nostalgia. They think seeing reruns in 2003 of shows from the 90s is the same as actually wearing the baggy jeans, riveted belt, and homemade tank top rocking your totally-not-natural blonde spiked hair while listening to Nirvana. Or locking yourself in your room full of Nick Carter and Joey Fatone posters, listening to BSB and *NSYNC after finishing up the recent Halloween episode of Boy Meets World.