What it is
Tiny random differences in amplitude between adjacent cycles (a percentage). It pairs with jitter: jitter tracks the wobble in timing, shimmer the wobble in size.
Where it comes from
Measure the peak amplitudes cycle by cycle and compute a statistic of adjacent-cycle differences.
How to read it
Healthy voices usually measure shimmer below 3–8% (depending on the measurement convention). Elevated values mean unstable amplitude control and often accompany breathiness and fatigue. Read the two together to locate whether the instability comes from the cycles or from the air.