What is the definition of nostalgia?

Nostalgic: a wistful desire to return in thought or in fact to a former time in one's life, to one's home or homeland, or to one's family and friends; a sentimental yearning for the happiness of a former place or time: a nostalgia for his college days.

Example: Paul was filled with nostalgia as he remembered the fun times in his past.

More input from Answers contributors:

• Nostalgia is the good feeling you get when you remember things from your past. When you kind of sit back and smile and think to yourself, wow, that was fun, or happy.

• Nostalgia is the act of longing for something past, for example, an ex-girlfriend, the country of origin, etc. It is when someone revives memories in his mind and feels sad that the "old times" are gone for good. (French Nostalgie).