What is the meaning of Fallen angel?

These corporate or government bonds were investment-grade when they were issued but have been downgraded by a rating service, such as Moody's Investors Service or Standard & Poor's (S&P). Downgrading may occur if the issuer's financial situation weakens, or if the rating service anticipates financial problems that could lead to default. The term is sometimes used more generically, too, to refer to stocks or other securities that are out of favor.