A dictionary is a collection of words and their definitions (and sometimes other information). Such collections are usually printed as books, but some are now designed for use on computers.
Dictionary originally came from the Latin word dictionarius, meaning "a manual or book of words." A dictionary is most commonly used to look up the definitions of particular words, but other information, such as etymology and usage guidelines, often appears as well. Electronic dictionaries can include even more information, like slang and popular texting acronyms, such as "ttyn," which stands for "talk to you never."