(countable)(linguistics) A consonant is a sound or letter that is not a vowel. Consonants usually do not last long and are less loud than vowels. In a syllable (a piece of a word) a vowel will be in the middle and consonants might come before or after.
The letters "t", "s", "f", and "g" are all consonants.
Your mouth is more closed when you say a consonant than when you say a vowel.