An evaluation from a rating agency such as S&P, Moody's or Fitch as to the relative strength of a bond. The highest rating indicates that there is miniscule risk of default, either as to interest or principal. Lower ratings indicate gradually greater default risks. In the S&P world, BBB is the highest rating for an "investment grade" bond. Anything lower than BBB is considered junk, er, high-yield. Caveat emptor.