Definition
Losing fat and gaining muscle over the same period, so the scale may barely move while the body changes.
It happens most readily in beginners, in lifters returning after a break, in people carrying more body fat, and in anyone whose previous programme was poor. Advanced lean lifters mostly have to pick one goal at a time.
The requirements: progressive resistance training three or more times a week, protein around 1.6 to 2.2 g per kilo of bodyweight, calories at maintenance or a small deficit, and sleep.
The scale is the wrong measure. Waist circumference, photos and the strength log tell the story; a flat weight next to a smaller waist and bigger lifts is recomposition working.