What is Zero consonant?

In orthography, a zero consonant, silent initial, or null-onset letter is a consonant letter that does not correspond to a consonant sound, but is required when a word or syllable starts with a vowel. Some abjads, abugidas, and alphabets have zero consonants, generally because they have an orthographic rule that all syllables must begin with a consonant letter, whereas the language they transcribe allows syllables to start with a vowel. In a few cases, such as Pahawh Hmong below, the lack of a consonant letter represents a specific consonant sound, so the lack of a consonant sound requires a distinct letter to disambiguate.