Alphabet

An alphabet was a system of letters.

Roman alphabet
The Roman alphabet was used in the English, French, and German languages. (PROSE: The Schizoid Earth)

There were twenty-six letters in the English alphabet: A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, V, U, W, X, Y and Z. (TV: The Masque of Mandragora, Midnight, Goodbye, Sarah Jane Smith, Shada, AUDIO: Shada, A Death in the Family)

While under mental attack by the Mandragora Helix, the Fourth Doctor and Sarah Jane Smith spoke the alphabet backwards to concentrate. (TV: The Masque of Mandragora)

When trying to remember "Young Parsons'" name, Professor Chronotis went through the alphabet with the Doctor and Romana II. They started with A, B, and C and eventually got to P, Q, and R before he skipped to X and Y. (TV: Shada, AUDIO: Shada)

In 2028, under the influence of Nobody No-One, a cardiogram once sung the alphabet from A to P. (AUDIO: A Death in the Family)

The Tenth Doctor and the entity possessing Sky Silvestry spoke the first fifteen letters of the alphabet simultaneously. (TV: Midnight)

Cyrillic alphabet
The Cyrillic alphabet common in Eastern European languages and used in Russian and Serbian. (PROSE: The Schizoid Earth)

Other alphabets
River Song, while unaware of the Twelfth Doctor's identity, threatened to remove their organs in alphabetical order, to which the Doctor facetiously asked "Which alphabet?" (TV: The Husbands of River Song)

Behind the scenes

 * UNIT Initiation Test, not considered a valid source on this Wiki, mentions the Dalek Alphabet, whose existence is known to UNIT.