What a habit is

Automatic, cued behaviour.

The loop

A cue triggers a routine which produces a reward, and repetition strengthens the link until the decision disappears. That automaticity is the definition.

Cues are usually time, place, preceding action, emotional state or other people.
Once automatic, the behaviour survives without the original motivation.

Good and bad

The mechanism is identical for brushing your teeth and for checking your phone. Nothing about it distinguishes helpful from harmful routines.