A consonant is a speech sound - not a letter - caused by a constriction or closure at some point along the vocal tract. Every syllable is understood to also contain a vowel sound but does not have to include a consonant.
English has 24 letters which largely correspond to pronounced consonants, but none of then has a completely consistent spelling. The English alphabet has no single letters corresponding to the consonant letters in bold in the following six words: chop; thank; shop; then; pleasure and thing.
 See also