Nowhere is this term defined, but everyone knows what it means.
It's like the instruction in the Talmud that after the wedding ceremony, the bride and groom shall "retire." Nobody asks what "retire" means; everyone knows what it means.Full faith and credit is the United States' unconditional guarantee to pay all interest and principal on every bond that it issues. It can make this guarantee because it can crank up the printing presses to Warp Factor 8 and literally print money to pay everybody back. Note that there's no guarantee that the money would actually be worth anything; you're only assured that you'll get pieces of paper in full payment of the obligation.