Noun. Late Middle English.
[Old French from Latin consonant-, consonans, present participle of consonare sound together, formed as CON-, SONANT.]
1(a) A letter of the alphabet that represents a consonantal sound. ME
1(b) An elementary speech sound other than a vowel,
in which the air-stream is at least partly obstructed,
and which in forming a syllable is usually combined with a vowel. E17
2 obsolete. = CONSONANCE. LME-E18
So, what is a consonant? In short, it's a basic unit of speech in which the sound is made using an obstructed or partially obstructed air-stream. This is clearly apparent in the P sound in a word like push - the very puh is made by obstructing the air and building up pressure behind your lips. But take the O sound in a word like over, and the whole syllable is completed with an expansive circle for your breath to breeze through unhindered. Therefore, O is clearly a vowel.
This is a very convenient distinction but, of course, English being English, it does rather like to muddle things up a bit. Is the letter Y, for example, a vowel or a consonant? Well, yes, obviously it's a consonant, because it's definitely not in the A-E-I-O-U list of vowels we learn in school. Technically, however, the answer is less clear, because in English Y functions as both a vowel and a consonant: in the word myth, for example, it behaves as a vowel, but in beyond it's clearly transformed itself into a consonant as it acts as an obstruction between the two vowels.
Whether or not Y is a vowel or consonant is really rather arbitrary, then. But, notes Oxford Dictionaries, as Y's consonant sound is not consistently represented by any other letter in English (unlike its vowel sound which can often be easily made with an I), this is probably the reason that Y is traditionally classed as a consonant.
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