A sparkline is a small, simple, data-dense graphic with typographic resolution that is shown inline and in-context with relevant text.
The term was coined by Edward Tufte, and inspired by Galileo's drawings of the shape of Saturn inline with his writings about them.
Contrary to the Wikipedia definition, sparklines are not limited to graphs, graphs are simply the most common use of sparklines (and "line graph" is unfortunately somewhat implied by the word "line" in the name). Rather, sparkline includes the kinds of graphics that inspired them in the first place, e.g. see Tufte's notes about Galileo's drawings of the shape of Saturn inline with text: