A statistic is a numerical piece of information. If you are trying to prove a point, and you want your argument to have a sense of authority, you might want to use a statistic to back up your claim.
A statistic is a single piece of information represented by a number. “Water makes up 70% of the Earth’s surface,” for instance, is a statistic. A statistic is like a fact that is reached through a mathematical process. A population figure is the most common statistic you will encounter. When made plural, statistics means either a collection of statistics or the science of creating and interpreting statistics, as in “His favorite course in college was Statistics.”