In articulatory phonetics, a consonant is a speech sound that is articulated with complete or partial closure of the vocal tract, except for [h], which is pronounced without any stricture in the vocal tract. Examples are [p] and [b], pronounced with the lips; [t] and [d], pronounced with the front of the tongue; [k] and [g], pronounced with the back of the tongue; [h], pronounced throughout the vocal tract; [f], [v], [s], and [z] pronounced by forcing air through a narrow channel (fricatives); and [m] and [n], which have air flowing through the nose (nasals). Most consonants are pulmonic, using air pressure from the lungs to generate a sound. Very few natural languages are non-pulmonic, making use of ejectives, implosives, and clicks. Contrasting with consonants are vowels.

Since the number of speech sounds in the world's languages is much greater than the number of letters in any one alphabet, linguists have devised systems such as the International Phonetic Alphabet (IPA) to assign a unique and unambiguous symbol to each attested consonant. The English alphabet has fewer consonant letters than the English language has consonant sounds, so digraphs like ⟨ch⟩, ⟨sh⟩, ⟨th⟩, and ⟨ng⟩ are used to extend the alphabet, though some letters and digraphs represent more than one consonant. For example, the sound spelled ⟨th⟩ in "this" is a different consonant from the ⟨th⟩ sound in "thin". (In the IPA, these are [ð] and [θ], respectively.)

The word consonant comes from Latin oblique stem cōnsonant-, from cōnsonāns 'sounding-together', a calque of Greek σύμφωνον sýmphōnon (plural sýmphōna, σύμφωνα).

Dionysius Thrax, a Classical Greek grammarian, called consonants sýmphōna (σύμφωνα 'sounded with') because in Greek, they can only be pronounced with a vowel. He divides them into two subcategories: hēmíphōna (ἡμίφωνα 'half-sounded'), which are the continuants, and áphōna (ἄφωνος 'unsounded'), which correspond to plosives.

This description does not apply to some languages, such as the Salishan languages, in which plosives may occur without vowels (see Nuxalk), and the modern concept of "consonant" does not require co-occurrence with a vowel.

The word consonant may be used ambiguously for both speech sounds and the letters of the alphabet used to write them. In English, these letters are B, C, D, F, G, J, K, L, M, N, P, Q, S, T, V, X, Z and often H, R, W, Y.

In English orthography, the letters H, R, W, Y and the digraph GH are used for both consonants and vowels. For instance, the letter Y stands for the consonant/semi-vowel /j/ in yoke, the vowel /ɪ/ in myth, the vowel /i/ in funny, the diphthong /aɪ/ in sky, and forms several digraphs for other diphthongs, such as say, boy. Similarly, R commonly indicates or modifies a vowel in non-rhotic accents.

This article is concerned with consonant sounds, however they are written.