A statistic (singular) is used to indicate the result of applying a statistical algorithm to a set of data. In the calculation of the arithmetic mean, for example, the algorithm directs us to sum all the data values and divide by the number of data items. In this case, we call the mean a statistic. To be complete in describing any use of a statistic, one must describe both the procedure and the data set.
The popular use of the term to mean a single measurement, or datum, differs from this meaning. A statistician would normally call an individual person's height a statistic only if that person were chosen randomly from some population of interest, but more often would use the term to refer to, for example, the median height of a group of people.