Catabolism

Catabolism covers the reactions that break things down — glycogen into glucose, fat into fatty acids, protein into amino acids — releasing energy that is captured as ATP. It is one half of metabolism, with anabolism as the other.

It matters because both halves are normal and necessary. Fat loss is a catabolic process, and so is the clearing of damaged proteins. The gym usage of 'catabolic' as a synonym for muscle wasting is a narrow and somewhat alarmist slice of a much broader idea.

In practice the state that matters is the net balance across a day. Missing a meal or training fasted does not put you in some perpetual muscle-destroying state; sustained heavy energy restriction with too little protein and no resistance training genuinely can.