"Rainism" was released in 2004 as the title track from Rain's fourth studio album. The song was a commercial success, topping various music charts in Korea. According to Rain, the song's lyrics were inspired by his own experiences with love and heartbreak.

Musically, "Rainism" is a fusion of genres, blending hip-hop, R&B, and pop elements. The song features a catchy, repetitive hook and a driving beat that propels the dance-oriented track forward.

The instrumental arrangement is noteworthy, with a combination of electronic and organic elements. The song's production features a prominent bassline, syncopated drum patterns, and filtered synthesizers that add texture and depth.

In an interview, Rain explained that "Rainism" is about the ups and downs of relationships. He wanted to capture the emotions and feelings that come with falling in love, but also the pain and sadness that can follow when things don't work out.