Medium literally means something in the middle. In this case it's a someone one who can be in the middle of people who are alive and in contact with deceased or non-human spirits. All Reiki Masters are capable of mediumship to a certain degree. One of the aspects of Reiki that, I believe, makes it so powerful is that Reiki practitioners are supported by a lineage. Each Reiki practitioner is connected to the lineage of Reiki Masters, most of whom have passed on from their earthbound incarnations.
I am often asked about contacting deceased loved ones using Reiki. Yes, this is possible, however it is not a good idea. Here's why, and there are two possibilities. When someone dies they begin a new journey as pure energy. Initially they are in an in-between state or in the Bardo (Bardo means in the between). This is, ideally, a very temporary situation. However, it is possible for spirits to be unable to progress and get stuck in the Bardo. This means that the spirit of the deceased person could be available to communicate through a Medium. If their death had a shocking quality by being premature, sudden, violent or unexpected, getting stuck in the Bardo has a higher probability. In those circumstances the living people who are connected to the deceased person are also prone to being in shock. This makes them more likely to have a desire to connect with the spirit of the deceased person. They are in need of appeasing their own grief and are concerned for the well being of the deceased person.
From the view of Tibetan Buddhism trying to contact a spirit in this circumstance is believed to be detrimental to the spirit of the deceased person. Once someone's earthly life is complete it is irrevocably final. However, their energy consciousness (because that is their pure state) can be pulled back to the people from their former lifetime that now is over. Not only is there no longer anything here for them, it can further distract them from their natural spiritual journey. By contacting someone who has died and is now in the Bardo you can actually hinder their spiritual progress.
After someone passes we continue to have an energy connection to the memory of that person and to their energy in their new state. They too have a connection to the people whom they have left behind. Again, if the nature of their death was shocking, a spirit's connection to earth life and people from their life may be strong. Though the sensation of receiving communication from a deceased loved one may give us a sense of temporary relief, I believe that it is important to recognize that it is ultimately a selfish action.
There is another possibility that is more likely. There is an energetic imprint of every event that has happened on earth. Actions, emotions and thoughts all leave an energy residue. When people with extraordinary abilities tap into this imprint they can "read" events from the past. Often, a medium can pick up information about a loved one who has passed because they are reading this energy. They are not in direct contact with the pure energy consciousness of the deceased. To hear about the genuine thoughts, emotions and intentions of someone we love can be comforting. However, it does not mean that the spirit or energy consciousness of the person who has passed is literally present. If the spirit is actually present, it is not a good sign.
So what is the take away here? When someone you care about passes, cultivate a deep knowing that they are on their own journey. The circumstances may not be what we want, expected or even like. The more love and compassion that you can direct to this understanding the more you will be able to support them. What you feel, what you think and what you do matters. If you are attached and clinging to their energy you can impede their journey. The ultimate, generous thing that you can do is to let them go. If you are angry, resentful or bitter you are still attaching to their energy. In this case, the ultimate, generous thing that you can do is still, to let them go ~ for yourself and for them.
If there is concern that the spirit may be stuck in the Bardo there are practices and rituals in many spiritual traditions to support those who have passed. In the Tibetan Buddhist tradition a practice called Phowa is utilized to aid the spirit of someone who has died transfer their consciousness from their earthly lifetime into the energy state of pure truth. Thus they are clear and unimpeded to progress as a spiritual being in their pure state. This practice is commonly done whether or not there is concern of the spirit being stuck in the Bardo.
There is a second part to these two scenarios. It is possible to communicate with beings in the spirit realm who have never been incarnate as a person on earth. From our view they are purely energetic beings. The issue is not whether or not this can happen. The issue and caveat is don't connect with just anyone*. Much as you, generally, don't connect with random strangers in an intimately, meaningful way. Be selective about who you connect with in the energetic realm. It's a good idea to connect with known or established energies that we are confident are wholesome and motivated by the highest good. It is not a good practice to be open to any random, energy being who* happens to be hanging around.
By spending time contemplating the impermanence of our lives we can move through the initial stages of fear and aversion to death. To recognize how potentially fragile and temporary our time here on earth may be we are better able to embrace our lives on a daily basis. This contemplation can bring clarity to our genuine heartfelt connections to others and help us to truly connect with those we love. I refer to Tibetan Buddhism a lot for two main reasons. First, they have maps for things that are not understood about our human condition in western culture. We don't even have words for much of it. And second, though they appear to spend a lot of time considering death, they are collectively the happiest group of people that I have ever encountered. Not frivously happy, but deeply, authentically filled with joy and compassion.
*not sure what the appropriate pronoun would be in this circumstance.