Definition of 'Investment Grade'
A rating that indicates that a municipal or corporate bond has a relatively low risk of default. Bond rating firms, such as Standard & Poor's, use different designations consisting of upper- and lower-case letters 'A' and 'B' to identify a bond's credit quality rating. 'AAA' and 'AA' (high credit quality) and 'A' and 'BBB' (medium credit quality) are considered investment grade. Credit ratings for bonds below these designations ('BB', 'B', 'CCC', etc.) are considered low credit quality, and are commonly referred to as "junk bonds".
Investopedia explains 'Investment Grade'
Investors should note that government bonds, or Treasuries, are not subject to credit quality ratings. These securities are considered to be of the very highest credit quality. In the case of municipal and corporate bond funds, fund company literature, such as the fund prospectus and independent investment research reports will report an "average credit quality" for the fund's portfolio as a whole.
Investors should be aware that an agency downgrade of a company's bonds from 'BBB' to 'BB' reclassifies its debt from investment grade to "junk" status with just a one-step drop in quality. The repercussions of such an event can be highly problematic for the issuer and can also adversely affect bond prices for investors. Safety-conscious fund investors should pay attention to a bond fund's portfolio credit quality breakdown.