An outlier is a particular measurement that is inconsistent with other measurements of a particular phenomenon.
For example, you might regularly compare the amount of fuel you put into a vehicle to the number of miles (or kilometers) that you have driven since the last fueling to monitor its fuel efficiency. In doing so, you will probably find that the values typically fall pretty close together: 28 mpg, 29 mpg, 28 mpg, 30 mpg, and so on. If one day you find that you got 18 mpg, that value is an outlier; because of its inconsistency, it says less about the fuel efficiency of the vehicle than the values that typically fall near 29 mpg. On the other hand, the outlier is probably worth investigating to determine what unusual circumstances caused such a dramatic drop in efficiency: it may indicate a mechanical problem, particularly unusual driving conditions during that tank of fuel, or even that fuel was siphoned from the tank at some point.
Outliers in Debates
It's not unusual for debaters to use outlier data points in debates as if they were performance standards for various technologies, even when the data point has dubious reliability. This can occur both to minimize opponents or to maximize favorites.