I could go on and on about this topic, sharing my personal experiences and information I have learned in courses/ training & research BUT i will give you the short version. A Medium is an individual that uses their natural ability to communicate information given to them by the means of energy/frequency AKA "Spirit". If your thinking "say what? thats crazy" don't worry, I was in your shoes ones upon a time! The idea of is seems impossible if you trying to think "logical", but if you follow along with some recent scientific theories and facts, you may be able to connect the dots a lot more clearly to satisfy that logical brain of yours!
In science we know that EVERYTHING is made of energy. We also know that everything has a frequency in which it moves for its molecular structure. We also know that energy can not be created or destroyed, it can only change forms (and frequency). So what does this have to do with being a medium? The answer is, it has everything to do with being a medium!
Think of a tray of ice cubes in your freezer. The energy in those frozen blocks of ice is lower right? The molecules are moving slower and as a result the water turns to a solid. Now, when you take the ice cubes out of the freezer, they start to melt. The molecules move faster and the kinetic energy increases. Now if you through them in a pot and crank up the heat (energy) the water begins to boil as a result of super crazy fast molecules darting in every direction and a huge increase of kinetic energy. These are all situations you see and feel physically right? (the ice is solid and cold, or the water is hot, makes bubbles and steam) these are all forms of energy and frequency. When the water evaporates into the air, you can't see it, but it there! So how does this connect with being a medium? The answer is pretty straight forward, just like people whom are more sensitive to light or sound than others, a medium/psychic is more sensative to the energy around them. They can see, feel, smell, or hear a larger range of energy/frequency around them such as beings that have changed "form".
You know that saying about animals "they can smell your fear"? Well it's kind of like that. When your afraid, your body releases a pheromone known as a "Human Alarm Pheromone". Basically you release or project energy that can be sensed by other human beings or animals. Some people in that situation react to the energy being projected and become more aware of their surroundings with a feeling of "something is wrong", just like an animal would, while others are very closed to their natural senses and will not pick up on that energy at all. Just like "good vibe" from a situation or a "bad vibe", you may feel that, but someone else will not.... it all has to do with how sensitive we are to energy.
So in the end of this "short version" of "What is a medium" it all comes down to these few basic things: A medium can connect to different frequencies of energy using their natural ability. A medium can connect to different frequencies because they are naturally more open to a greater range of frequency. A medium receives messaged on different levels of frequency and can interperate them using skills they have developed.
I hope this helps!
Light & Love,