The simple answer is it’s all about usage.
The exact process depends on the dictionary. Not all dictionaries have the same words in them but the process is similar for each.
Each day, dictionary editors read through books, newspapers, electronic publications, etc., looking for new words, new word usages, variants, etc. All words of interest are marked, along with any surrounding context that offers insight into its form and use.
Once a new word or phrase has been marked, editors enter it into a database and create a citation for the word. This citation includes three things:
1. The word or phrase.
2. An example of the word or phrase used in context.
3. Bibliographic information about its source.
At this point, these words aren’t guaranteed inclusion into the dictionary. They are simply contestants, per se. Before a new word can be added to the dictionary, editors have to find enough citations to prove it is being widely used. Even a single word having numerous citations doesn’t guarantee inclusion into the dictionary. If citations do not provide a clear definition of the word or if all the citations come from a single source, it may be rejected.
If a word has enough unique citation, it will be approved and added to the dictionary.