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, Serbian, (PROSE: The Schizoid Earth) and Uzbek. (AUDIO: Brave New Town)

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)

Greek alphabet
Letters in the Greek alphabet included: Η, Θ, Π, Ρ, and Σ. (TV: The Five Doctors, AUDIO: Trial of the Valeyard)

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.