A dictionary was a book of definitions for various words, ordered alphabetically. One such dictionary was the Oxford English Dictionary, which defined English language terms. (PROSE: Observation) Dictionaries could also provide synonyms. (PROSE: The Stars Our Contamination)
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)
Fitz Kreiner speculated that "utilitarian" and "functional" probably weren't even in the Yquatine dictionary. (PROSE: The Fall of Yquatine)
Bilingual dictionaries also existed, which allowed the owner to quickly find a word in another language. The Tenth Doctor had a pocket Gallifreyan-Cymraeg dictionary. (PROSE: The Eyeless)