Vowel

Vowels were a type of letter.

As the Sixth Doctor noted, (AUDIO: The Carrionite Curse ^[+]Loading...["The Carrionite Curse (audio story)"]) all five vowels were present in the word facetious, which Agorax answered correctly on The Weakest Link. (TV: Bad Wolf)

In the year 2000, the Cybermen released a computer virus that destroyed all the vowels. (PROSE: Vrs)

According to Donna Noble, vowels were a relatively easy type of letter compared to other kinds for a person with a stutter to enunciate. (TV: Forest of the Dead)

The "dent" left on the English language when Nobody No-One's CORDIS crashed into it was noticed by English-speaking humans and referred to as "The Great Vowel Shift". (AUDIO: A Death in the Family)