what is the legal definition of subcontracting?

Answer 1

Subcontract is a contract between a party to an original contract and a third party that assigns part of the performance (as building a house) of the original contract to the third party

For example: Home builder needs roofing/gutter work completed, he hires a roofing company to complete the job. The roofer will hire his own crew to hang gutters.

Answer 2

A person who is hired by a contractor, paid directly by the contractor but still working on a project that the contractor has the contract on. Not a legal definition, but close enough. As a sub contractor you still have "rights".