A consonant is a unit of language, defined in phonetics as a speech sound that involves full or partial 'closure' of the oral tract (the mouth), preventing or restricting airflow, and in phonology as a segment that cannot occupy the nucleus or 'peak' of a syllable. Consonants, then, are not just speech sounds, but abstract entities used to build up larger structures, e.g. syllables, within an utterance; they can therefore appear in written language (e.g. <B>, <C>, etc.) as well. Consonants are similar to vowels in this regard, but different terminology is used to describe their articulation. Their production usually involves the tongue, but the lips, vocal folds, velum and the nasal cavity may also be involved, and the direction of the airflow itself (the type of airstream mechanism) is also important.
Phoneticians describe the articulation of consonants in terms of: the place of articulation (the position of the active articulator, i.e. the tongue or lower lip, relative to the passive articulator, e.g. the roof of the mouth, from the front towards the back of the oral tract); the manner of articulation (the vertical distance between the active and passive articulators); the presence or absence of voicing (vibration of the vocal folds); the position of the velum (a muscular flap towards the back of the mouth, which can block air from entering the nasal cavity); and the type of airstream mechanism itself (the most common being pulmonic egressive, where all sounds are produced with air from the lungs). For example, [p] is bilabial (place, i.e. articulated with the lower lip raised to make contact with the upper lip), plosive (manner, complete obstruction), voiceless (no vibration of the vocal folds as air passes between them), oral (the velum is raised to prevent air escaping through the nasal cavity), pulmonic (the air comes from the lungs) and egressive (the air moves from the oral tract outwards). It differs from [b] only in that [b] involves vocal fold vibration; from [t] only in that the place of articulation is now alveolar (contact between the tongue and the area just behind the top front teeth; and [p'] (an ejective, not really found in English) only in that the airstream is now glottalic (airflow is initiated through the glottis closing and rising) instead of pulmonic. By changing each of these features, any sound of any language can be described.