Concept:
Threshing performance is evaluated by different losses such as cylinder loss, blower loss, sieve loss and grain damage.
Step 1: Un-threshed grains are grains that remain attached to the ear head or panicle after threshing.
Step 2: These grains indicate incomplete threshing.
Step 3: Incomplete threshing is mainly associated with the threshing cylinder.
Step 4: Therefore, the percentage of un-threshed grains from all outlets with respect to total grain input is called cylinder loss.
\[
\boxed{\text{Cylinder loss}}
\]