The Median is the middle of a ordered distribution of a sample, range, population, or simply list of numbers or values. When this distribution has an odd number of terms, the median is the middle most value. If the number set is even then the median is the mean of the two middle values. In contrast to the mean, the “average” that the median represents is less sensitive to outliers. If a point is added to a sample far from where the data clusters the median will shift only in terms of the total number of points in the sample and not be effected by the value of that outlier. The median as the middle value represents the divide between the lower half of the range and the upper half.
The wikipedia article on the median is extensive but generally contains more sophisticated and technical information beyond the needs of standard statistics. The University of Michigan has a helpful open textbook with a brief but useful article on not only the median, but most of the introductory topics and definitions relative to statistics. A more formal definition of the median can be found on the Wolfram Alpha median page.