* New words: New words are constantly being created, and it takes time for them to be added to the dictionary.
* Slang: Slang words are often not considered to be "proper" English, and are therefore not included in the dictionary.
* Technical terms: Technical terms are often only used in specific fields, and are therefore not widely known enough to be included in the dictionary.
* Foreign words: Foreign words may not be included in the dictionary if they are not commonly used in the English language.
* Offensive words: Offensive words may be excluded from the dictionary in order to avoid offending readers.
It is important to note that just because a word is not in the dictionary does not mean that it is not a "real" word. Dictionaries are simply a record of the words that are most commonly used in a language.