Which Buddha do Buddhists take refuge in?

Does one take refuge into the concept of Buddhahood or refuge into Gautama Buddha? What about Vairocana Buddha or Amitābha Buddha for that matter? I've heard various things from taking refuge in a cosmic Buddha principal to the actual Gautama Buddha over the years from various Buddhist teachers. Perhaps taking refuge into the Buddha is a relative subjective experience. Which Buddha do Buddhists take refuge in?