It is used when something sparks some good feeling/memory from the past. It's like a reaction after something that brings back good memories. 'Those pokemon cards are so nostalgic!' (They bring back so many good memories) '*Walking into an old record shop* There is so much nostalgia here!' 'These clothes make me feel nostalgic'

Nostalgia is like a feeling. It can be a good feeling about someone or something or It also can be a bad feeling. Do you know the word "saudade"? Nostalgia is when you feel "saudade" (you miss something or someone) and you remember that and think about that. So, you are "nostálgico".

Thanks. Second example is good.

When you remember something longingly. If it's a sad memory, you look back on it and it makes you seem "wistful". If you remember happy times in the past, you "take a stroll down memory lane". First example: "Nintendo taps into the power of nostalgia" (Irish Times, 28 July 2016) = '90's nostalgia Second example: Once a week, the internet engages in a form of organised collective nostalgia known as Throwback Thursday where people post throwback pictures on Facebook.