The yield difference between speculative-grade (junk) corporate bonds and US Treasuries, measuring market credit risk perception. • Below 300bp: Optimistic market, low credit risk • 300–500bp: Normal range • 500–800bp: Rising stress, slowdown concerns • Above 800bp: Crisis levels (2,000bp in 2008, 1,100bp in 2020) Widening spreads mean investors demand higher compensation for risk, often leading equity market declines. Considered a 'smart money' fear indicator.