The 30-year Treasury bond is the longest-maturity Treasury security. Its closest rival is the 10-year Treasury note.
Through the end of the 20th century, the "long bond" (or even just "the bond") was the U.S. benchmark, meaning that people looked to its yield as a proxy for all U.S. interest rates. Then, reductions in issuance of 30-year bonds gave them a scarcity value, making them less reliable as an indicator of how high people thought interest rates should be. Now, the 10-year note now serves as the U.S. benchmark.