What is (a) A sound intermediate between a vowel and a consonant, or partaking of the nature of both, as in the English w and y. (b) The sign or letter representing such a sound called?

semivowel — (a) A sound intermediate between a vowel and a consonant, or partaking of the nature of both, as in the English w and y. (b) The sign or letter representing such a sound. (Webster's 1913.)

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