In a set of numbers that measure something — test scores, heights, prices, temperatures — an outlier is the one that sits far away from all the others: much higher, or much lower. If nine students score between 62 and 78 and one scores 12, that 12 is the outlier. Scientists and students use it when they draw a graph: the lonely dot off to the side. The word is a little technical, so you meet it in maths, science, and news stories about data, but it has also wandered into ordinary talk. Though actually, when people say someone is an outlier they often mean a person who does not fit the group, not a number. That is a stretch of the same picture — lying outside the cluster — and you will hear it. This entry is the number sense, the one from the classroom. People sometimes drop an outlier before they take an average, because one wild number can pull the average into a lie; that can be wise, and it can also hide a real result, so the careful habit is to look at why it is so far away first.