Taken from Wikipedia, this is their definition:
The term "mediumship" denotes the ability of a person (the medium) to experience what they or others believe is contact with spirits of the dead, angels, or other immaterial entities. The role of the medium is to attempt to facilitate communication with spirits who have messages to share with non-mediums. Mediums claim the abilities to listen to and relate conversations with spirit voices; go into a trance and speak without knowledge of what is being said; allow a spirit to control their body and speak through it; relay messages from the spirits to those who wish to contact them with the help of a physical tool, such as a writing instrument. Mediumship is also part of the belief system of some New Age groups. In this context, and under the name channelling, it refers to a medium who claims to receive messages from a "teaching-spirit".
This is my explanation of mediumship:
Mediumship is the ability to communicate with spirits who have crossed over into the spirit world, or heaven. Sylvia Browne, John Edward, Lisa Williams and James Van Praagh and Theresa Caputo are psychic mediums. You might have seen them on television. Psychic medium Theresa Caputo has a television show called "Long Island Medium". Mediumship can be done through several forms of communication. A medium may sense a spirit, feel what they looked like or sense what type of personality they had. This is called clairsentience.
Spirit can sometimes be seen with physical eyes, or most commonly through the mind's eye, or third eye. It's actually like replaying a memory in your mind, or daydreaming. This is called clairvoyance.
Hearing spirit is obviously another way of communicating. Spirit can be heard sometimes in the ear, or, more likely than not, in the mind. Hearing spirit is just like talking to yourself, or thinking to yourself. I am clairvoyant, clairaudient and clairsentient.
Mediums have to raise their vibration to communicate with spirit while at the same time, spirit will lower theirs a bit so we can connect.
Although a psychic medium reading is not a cure for grief, it can help ease the pain and bring some relief just knowing your passed loved one is okay. I am the middle person (medium) who receives information and messages from your passed loved ones that can help answer your questions about the afterlife, and assure you that life is eternal.
The spirits provide evidence so that you will recognize it is your loved one communicating as their personality comes through. You will know by their unique sense of humor, serious or even bossy tone and have confidence they are in fact communicating with us during your session. A lot if times, additional spirits may come through to send a message or just to say, "Hi, I''m here and I'm okay".
All mediums are psychic. Not all psychics are mediums. If you decide to get a reading with a medium, you will be able to hear from your passed loved ones and also receive psychic information pertaining to your current life situation and also your future.