Quarter turn metric
The quarter turn metric (QTM) is a metric for the 3x3x3 where any turn of any face, by 90 degrees clockwise or counterclockwise, counts as 1 turn; thus it is different from the half turn metric because half turns count as two moves instead of one. It is important to note that in QTM a slice move can count as either two turns (if it is a quarter turn like M) or four turns (if it is a half turn like M2), since the centers are assumed to be fixed. Cube rotations do not count as turns, so a 'double layer' turn such as r would still count as one turn.
Thus the following algorithm would count as 12 turns QTM: r2 U R' U' M U R U' R' r' (an edge 3-cycle on the front face).