|This page in a nutshell: In Wikipedia, things are grouped into articles based on what they are, not what they are called by. In a dictionary, things are grouped by what they are called by, not what they are.|
Wikipedia is not a dictionary, phrasebook, or a slang, jargon, or usage guide. Instead, the goal of this project is to create an encyclopedia. Our sister project Wiktionary has the goal of creating a dictionary. It is the "lexical companion to Wikipedia", and the two often link to each other. Wiktionary welcomes all editors who wish to write a dictionary.
Both dictionary entries at Wiktionary and encyclopedia articles at Wikipedia may start out as stubs, but they are works in progress, to be expanded. Wikipedia articles should begin with a good definition, but they should provide other types of information about that topic as well. The full articles that Wikipedia's stubs grow into are very different from dictionary entries.
Each article in an encyclopedia is about a person, a people, a concept, a place, an event, a thing, etc., whereas a dictionary entry is primarily about a word, an idiom, or a term and its meanings, usage and history. In some cases, a word or phrase itself may be an encyclopedic subject, such as Macedonia (terminology) or truthiness.
One perennial source of confusion is that a stub encyclopedia article looks very much like a dictionary entry, and stubs are often poorly written; another is that some paper dictionaries, such as "pocket" dictionaries, lead users to the mistaken belief that dictionary entries are short, and that short article and dictionary entry are therefore equivalent.