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.<Essentials of Linguistics (2nd edition) by Catherine Anderson and others>[§3.3, bilabial place and the lower lip as active articulator]{ref}

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>.


 Bilabial consonant (source code)