Qualifying institutional buyer
Wikipedia has a pretty good entry on QIBs:
https://en.wikipedia.org/wiki/Qualified_institutional_buyer
This concept is relevant to US persons who are purchasing debt securities in offerings which are not SEC registered. Generally US persons cannot buy from public offers of securities that are not registered with the SEC (known as Regulation S issues - it includes most Eurobonds issued in the London market.
There is an exemption - the Rule 144A exemption, which applies to private offers of securities to QIBs. QIBs are basicvally "big boys" who do not require SEC protection to make these investments. They must hold the securities for a certain period before selling them.
Not to be confused with TEFRA rules relating to bearer securities, which are tax related.