What's "nostalgia?" Is it like a memory?

Native speaker’s answer
Rebecca
"Nostalgia" is a sentimental longing or affection for a period in the past. It comes from our memories, but it is not just a memory as it describes the feeling that our memories give us. Ex: A wave of nostalgia swept over me when I saw my childhood home. Ex: He was filled with nostalgia for his college days.