  We report on the coexistence of liquid-like and gas-like structures in
supercritical fluid (SCF). The deltoid coexistence region encloses the Widom
line, and may therefore be termed the "Widom delta". Machine learning analysis
of simulation data shows continuous transition across the delta, from
liquid-like to gas-like states, with fractions following a simplified two-state
model. This suggests a microscopic view of the SCF as a mixture of liquid-like
and gas-like structures, where the anomalous behavior near the critical point
originates from fluctuations between the two types.
