Definition 1
“Consonant sounds are made by restricting or blocking the
air flow in soe physical way, and this restriction, or release
of the restriction, is what gives the consonant its
characteristic sound. By contrast, vowels require the vocal
tract to be open so that the air stream escapes
unobstructed”.
(…)
Definition 2
“Consonants, either singly or in clusters, mark the
beginnings and ends of syllables. Vowels occur as the
centres or focal points of syllables, either between
consonants or their own”.
(UNDERHILL, A. Sound Foundations. Oxford:
Macmillan, 1994, p. 29).