Sunshine pop (originally called "soft pop") is a lightly-produced pop style influenced by commercial jingles and easy-listening, usually incorporating acoustic guitars, vocal harmonies, bossa nova rhythms, and (occasionally) lush orchestras. It originated from Southern California in the mid 1960s, and was most prominently represented by the Association, the Mamas & the Papas, and the 5th Dimension.
The genre is often erroneously thought of as "Beach Boys-style music" or "music with sunny themes". Although the Beach Boys had a large role in forming the bedrock for sunshine pop, they could rarely be classified in the genre. Additionally, a high percentage of sunshine pop is melancholic, anxious, or borderline-psychedelic, far-removed from any "sunny" cheerfulness. Most acts who were associated with the genre were imitations of more popular groups, largely producer-driven, and riding on the coat-tails of the nascent folk rock/folk pop scene. For these reasons, they're better thought of as precursors to the adult-contemporary/soft rock groups of the early 1970s (i.e. the Carpenters).
After the early 1970s, sunshine pop quickly fell by the wayside. The reissue culture and lounge revivalism of the 1980s-90s brought something of a resurgence in the genre, influencing various indie pop circles related to chamber pop or Shibuya-kei. .