From Wiki: "A PLUS Loan is a student loan offered to parents of students enrolled at least half time, or graduate and professional students, at participating and eligible post-secondary institutions. The original, now obsolete, meaning of the acronym was "Parent Loan for Undergraduate Students"."
And see this article: http://www.forbes.com/sites/laurengensler/2016/09/07/how-some-millennials-end-up-paying-their-parents-student-loan-debt/#5252a08c449f