Principal Applicant

The person named in the petition.  For example, an American citizen may file a petition for his married daughter to immigrate to the United States.  His daughter will be the principal applicant, and her family members will get visas from her position.  They will get derivative status.  Or a company may file a petition for a worker.  The worker is the principal applicant.  Family members get derivative status.

Source: U.S. Department of State.