A bilabial consonant uses the two lips to form its principal constriction. The term specifies place of articulation; it leaves manner of articulation and voicing to be stated separately. The lower lip approaches the upper lip rather than the upper teeth.[ref]
Essentials of Linguistics (2nd edition) by Catherine Anderson and others. §3.3, bilabial place and the lower lip as active articulator
The sounds
[p], [b] and [m] share a bilabial closure. The first two are oral stops, while [m] permits nasal airflow; their shared lip position therefore does not determine whether sustained sound is possible during closure. A mirror provides direct evidence of the lip configuration, but it cannot alone establish the state of the vocal folds or the soft palate. New to topics? Read the docs here!