Fat free mass

Everything in your body that is not fat: muscle, organs, bone, and all the water they hold.

Your resting energy use is built on it, and preserving it is most of the difference between losing weight well and losing weight badly.

It is mostly water by weight, so it moves with hydration far more than real tissue does. A kilogram lost overnight is water, not muscle. It equals soft lean mass plus bone mineral content.

Worth mentioning a steady decline, particularly if you are also losing strength.