Nostalgia is a complex emotion that combines LONGING, REGRET, and HOMESICKNESS. It is a bittersweet emotion that is often associated with memories of the past. LONGING is a feeling of yearning or desire for something that is absent or unattainable. REGRET is a feeling of sorrow or remorse for an action or decision that cannot be changed. HOMESICKNESS is a feeling of sadness or LONGING for one's home or homeland. Together, these three emotions create the complex emotion of nostalgia.