everyday chemistry - Why is mercury used in thermometers?

I know it expands when hot and grows compact when cool (that's about all I know), but I suppose many other substances do, too.

Also, and excuses for my ignorance, what the term used to describe the function of mercury in a thermometer? I mean, would you call it an "indicator" (insofar as it indicates the temperature), or is there some technical term?

Thanks!