A sensitive is a person who has the ability to detect fluctuation in energy around them. This means they may know when a spirit is around, whether the person they're with has a change in their mood, or whether or not a person or place is good (or bad) to be around. They may be able to use their abilities to communicate with spirits and/or predict the outcome of a particular situation based on their own feelings.
That's my working definition. I'm sure others on here can elaborate.