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)

Letters in the Russian alphabet included: Б, В, Г, Д, Е, И, Й, К, Л, М, Н, О, П, Р, С, Т, Х, Ш, and Ь (TV: Before the Flood, PROSE: Emotional Chemistry)

In Cyrillic, the USSR was abbreviated as СССР. (TV: The Seeds of Death, PROSE: First Frontier)

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.