A transitional sound in speech which is produced by the
   changing of the mouth organs from one definite position to another, and
   with gradual change in the most frequent cases; as in passing from the
   begining to the end of a regular diphthong, or from vowel to consonant
   or consonant to vowel in a syllable, or from one component to the other
   of a double or diphthongal consonant (see Guide to Pronunciation, //
   19, 161, 162). Also (by Bell and others), the vanish (or brief final
   element) or the brief initial element, in a class of diphthongal
   vowels, or the brief final or initial part of some consonants (see
   Guide to Pronunciation, // 18, 97, 191).