Estrogens or Oestrogens, are a group of compounds named for their importance in both menstrual and estrous reproductive cycles. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal. The name comes from the Greek οἶστρος (oistros), literally meaning “gadfly” but figuratively sexual passion or desire, and the suffix -gen, meaning “producer of”. Estrogens are synthesized in all vertebrates as well as some insects.