An Ascended Master is a Being who has paid all of their karma, left the cycle of rebirth and attained Mastery over themselves through their own endeavors and hard work.
An Ascended Master is a Being of Light who is not in body. An Ascended Master has had many lives in a physical body, just like you. Through these many lives, their own freewill, and hard work, they raised their vibration and consciousness to such a state that they cleared all of their karma and dross from their chakras and bodies. The Ascended Master is not the past life person, as it is not the lower-self person who ascended. In truth, the Ascended Master is the Higher Self of the person who strived to ascend. So, when you call on Jesus, you are calling on the past life persona of the person who ascended to and surrendered to his Higher Self. It is not the persona of Jesus that responds, rather it is the Higher Self of Jesus that responds.
We all have a Higher Self. All of the Higher Selves of those striving to ascend are Masters in waiting. All Higher Selves are great Beings who continually strive to become a Being of total purity and Light. They continually work on their own ascension process.
There are many Ascended Masters, some recent and others ancient, some whose names we recognise and some whose names aren’t in our conscious awareness. Some so old, they are mythical to us now. Regardless of who they were in their countless past lives, they are returning now to help us in our own ascension process, for this is the time for many to ascend.
You can read more about some of the Ascended Masters individually here: Meet the Ascended Masters
As you can see, this list of Ascended Masters is by no means a complete list, as many Masters are beyond my awareness. The Masters on this list are the Masters I can verify as Masters. I have seen many of them in the Ascended Masters’ portal.