One definition of an ascended master is an individual who has undergone the process of ascension. Throughout history, there have been stories of these individuals who have reached a higher state of spiritual awareness and placed themselves in service to humanity. One concept of an ascended master derives from the teachings of Theosophy.
In Theosophy, and various descendants and offshoots of theosophy, ascended masters are a group of spiritually enlightened beings, once mere mortals, who have undergone a process of spiritual transformation. According to these teachings, they remain attentive to the spiritual needs of humanity, and act as superintendents of its spiritual growth. In this, they can be compared to the Great White Brotherhood or Secret Chiefs who are posited by various magical organizations; and more remotely, to the bodhisattvas of Buddhism, or the saints of Catholic and Orthodox Christianity.
The Great White Brotherhood
Belief in the Brotherhood and the masters is an essential part of the syncretistic teachings of these several groups. Various important spiritual leaders such as Jesus, the Prophet Muhammad, the Virgin Mary, and Kuan Yin the compassionate bodhisattva, take their seats alongside magical or alchemical personalities like the Count of St. Germain, and other mystic celebrities like Kuthumi, one of Helena Petrovna Blavatsky's claimed spiritual guides - all of these leaders have put aside any differences they might have had in their Earthly careers, and unite instead to improve the spiritual well-being of the human race.