A 'consonant' is Latin for 'sounding together' — named because it can only be heard with a vowel.
A speech sound produced by partially or fully obstructing airflow through the vocal tract; a letter representing such a sound.
From Old French 'consonante,' from Latin 'cōnsonāns' (sounding together, harmonious), the present participle of 'cōnsonāre' (to sound together, to be in agreement, to harmonize), a compound of 'con-' (together, with, from PIE *kom) + 'sonāre' (to sound, to make a noise, to resound), from 'sonus' (sound, noise, tone), from PIE *swenh₂- (to sound, to resound, to make noise). PIE *swenh₂- gives Latin 'sonāre' (sound), 'sonus' (sound), 'sonitus' (noise), and English 'sound,' 'sonic,' 'sonata,' 'sonnet,' 'resonance,' 'dissonance,' 'consonance,' and 'unison.' The Romans named consonants ('litterae cōnsonantes') as letters that 'sound together' — they can only be voiced together
'Consonant' and 'vowel' are mirror concepts named by the Romans: a 'vowel' (vōcālis) is a 'vocal' letter that sounds on its own, while a 'consonant' (cōnsonāns) 'sounds together' with a vowel — it cannot be fully pronounced alone. The names encode a dependency: every consonant needs a vowel, but not vice versa. The same PIE root *swenh₂- gave us 'sound,' 'sonic,' 'sonata,' and 'resonate.'