The Option-Adjusted Spread on the ICE BofA US High Yield Master II Index. Measures the extra yield high-yield (junk) bonds pay over Treasuries.
HY credit spreads are one of the most sensitive live indicators of financial stress. They widen before equity markets sell off and tighten during risk-on recoveries.
Below 300bps = tight, risk-on. 300-400bps = caution. 400-600bps = stress. Above 600bps = crisis. Spreads widened to 2,000bps in March 2020.