The Fed has adopted quantitative thresholds for its monetary easing measures.

Rates will stay low as long as inflation is at 2.5 percent or less and as long as the unemployment rate is above 6.5 percent.

The Fed also announced a fourth round of quantitative easing, "QE4," to replace the expiration of Operation Twist at year-end.

The Federal Reserve will continue purchasing $45 billion per month of long-dated Treasury bonds as it did under Operation Twist, but without sterilizing the purchases with the sale of short-term notes.

QE4 was not a surprise, but the adoption of quantitative thresholds was not viewed as likely until sometime in 2013.