It is the sentimentality of our past, usually for a particular time and place associated with positive emotions, etched in our memories. Historical texts state it was termed as homesickness during war times.

It is now considered a natural, common emotion, a way to escape from the current 'space-time' and mentally travel to one's past when the world around us was different. It is usually prompted by a feeling of loneliness, disconnectedness or meaninglessness, triggered by thoughts about the past, by our senses of smell and touch or through music or weather.