A dictionary was a book of definitions for various words, ordered alphabetically. When General Cobb told the Tenth Doctor that genocide and peace in our time was the same thing for him, the Doctor sarcastically suggested that he should get a better dictionary and look up genocide. The Doctor told Cobb that he would find a picture of him there, and the caption would read "Over my dead body!". (TV: The Doctor's Daughter)
Bilingual dictionaries also existed, which allowed the owner to quickly find a word in another language. The Tenth Doctor told Martha Jones that he owned a pocket Gallifreyan-Cymraeg dictionary, despite never having learnt the Welsh language itself. (PROSE: The Eyeless)