The ratio of mean coil diameter to wire diameter in a spring.
Spring index governs whether a spring is manufacturable and how it behaves. Below about 4 the coils are too tight to wind and stress concentrates severely on the inside of the coil. Above about 12 the spring is floppy and prone to buckling. Six to ten is the comfortable range.
It appears in the Wahl correction factor, which accounts for direct shear and for the stress concentration at the inside of the coil — the point where a spring actually fails.
Designing a spring with a very low index for a high rate. It cannot be wound reliably and will fail at the inside of the coil.
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