Here is the most succint definition of an ascended master I have yet found (See link below):
Ascended Master - a being who has become Self-Realized and serves humanity; a
being who has raised his/her vibration to a sustained frequency of light. He/she
can come and go at will from the earth plane without the Birth//Death cycle.
Here is more speculative information on the Great White Brotherhood (?) aka The Great White Lodge.EL MORYA
- The Great White Brotherhood is the school of the Great White Lodge according to the AMORC Rosicrucian order
- Every student or adept aspires to Great White Brotherhood membership
- Note the comments below of Bulgarian Gnostic master Peter Deunov
- And the significance of Omraam Mikhaël Aïvanhov (Deunov's principal disciple)
- who established Fraternité Blanche Universelle still operating today
- Guy Ballard's "I AM" activity
- The Brotherhood of "Immortal Saints and Sages" popularized by Ascended Master Teachings developed byThe Bridge to Freedom, The Summit Lighthouse, and The Temple of The Presence. 
- Isaac Bonewits Neo Pagan and critical of the Manichaean symbolism of white and black. (1971) book Real Magic
Remember some of the ascended masters remained behind to teach student/adepts and advance their evolving spirituality?
EL MORYA was one such:
A Teaching Master
El Morya Chohan of the First Ray,
connected to the Temple of the Will of God. El Morya was embodied as Abraham, the wise man
Melchior, Arthur, king of the Britons, Thomas
Beckett, Thomas More, both martyred, Akbar, greatest of Mogul emperors,
Thomas Moore, El Morya Khan, the most renowned of the Tibetan
mahatmas. El Morya
came originally from Mercury and is a member of the White Brotherhood. He works with Archangel Michael and is the chohan or master of the
first ray, which is red and governs power and will, drive, confidence and
strength. He works with rulers, executives, public servants, the military,
sports people and those who are commanders.El Morya - Blue Ray - Throat Chakra. Working to transform the aggressive human
from domination to the will to do good. Head of all Esoteric Schools.
Abraham The patriarch Abraham, "father of many nations," was called
the "Friend of God." The LORD God called Abraham out of Ur of the Chaldees and promised Him
"I will make of thee a great nation." Abraham is now the Ascended Master El Morya.
Bulgarian Gnostic master Peter
Deunov referred to his organization of followers as the "Universal White
Brotherhood," and it is clear that he too was referring to the Western esoteric
community-at-large. When ex-communicated as a heretic on 7 July 1922, he defended the Brotherhood
‘Let the Orthodox
Church resolve this issue, whether Christ has risen, whether Love is
accepted in the Orthodox Church. There is one church in the world. But the
Universal White Brotherhood is outside the church - it is higher than the
church. But even higher than the Universal White Brotherhood is the Kingdom of
Heaven. Hence the Church is the first step, the Universal White Brotherhood is
the second step, and the Kingdom of Heaven is
the third step - the greatest one that is to be manifested.’ (24 June 1923).
Similarly, Macedonian teacher Omraam Mikhaël
Aïvanhov (Deunov's principal disciple)
formally established Fraternité
Blanche Universelle as an "exoteric" esoteric organization still operating
today in Switzerland, Canada, the USA, the UK and parts of Scandinavia.
The term Great White Brotherhood was further developed
and popularized in 1934 with the publication of "Unveiled Mysteries" 
Ballard's "I AM" Activity.
This Brotherhood of "Immortal Saints and Sages" 
who have gone through the Initiations of the Transfiguration, Resurrection, and
was further popularized by Ascended Master
Teachings developed byThe Bridge to
Freedom, The Summit
Lighthouse, and The Temple of The
Other esoteric traditions, however, dismiss or deny the existence of a Great
White Brotherhood. Neo-Pagan author Isaac
Bonewits has been sharply critical of the concept; in his 1971 book Real
Magic, Bonewits dismisses the "Great White Brotherhood" as part of his
rejection of what he sees as the Manichean dualism of the implied
opposition of whiteness and darkness, white magic versus black
magic. Bonewits mentions the Brotherhood as an example of his larger thesis,
that "(t)he whole idea of White as Good and Black as Evil is purely the result
of cultural bigotries."
Please also refer to the book Cosmic Consciousness By M. Bucke.
Only a few of this community are known to the human race; these initiates
are the "teaching Masters."The AMORC Rosicrucian order maintains a
difference between the "Great White Brotherhood" and the "Great White
Lodge",saying that the Great White Brotherhood is the "school or fraternity" of
the Great White Lodge, and that "every true student on the Path" aspires
to membership in this Brotherhood.Some
of Aleister Crowley's remarks appear to indicate that Crowley identified the
Great White Brotherhood with the A∴A∴, his magical secret