Slice Turn Metric
The slice turn metric (STM) is a metric for the 3x3x3 where any turn of any layer, by any angle, counts as one turn. This differs from HTM in that a slice move counts as one turn, not two. And it differs from the ATM in that it does not include anti-slice turns and other axial turns.
The following algorithm would count as 10 turns STM: r2 U R' U' M U R U' R' r' (an edge 3-cycle on the front face).
God's Number in Slice Turn Metric is between 18 and 20 moves, but the exact number has not yet been proven.