I think it's a self-employed person who may carry out work for various companies, but is not actually employed by them or on their payroll. I think many companies like this sort of arrangement, as they don't have to pay a contractor any holiday entitlements, sick pay, pension contributions, etc. That's pretty much how it works in the UK, as far as I can gather. Not sure whether all this applies in the USA though.
_________________
On a mountain range
I'm Doctor Strange