The word word has a wide range of meanings and uses in English. Yet one of the most often looked for pieces of information regarding word is not something that would be found in its definition. Instead, it is some variant of the question, What makes a word a real word?
One of the most prolific areas of change and variation in English is vocabulary; new words are constantly being coined to name or describe new inventions or innovations, or to better identify aspects of our rapidly changing world. Constraints of time, money, and staff would make it impossible for any dictionary, no matter how large, to capture a fully comprehensive account of all the words in the language. And even if such a leviathan reference was somehow fashioned, the dictionary would be obsolete the instant it was published as speakers and writers continued generating new terms to meet their constantly changing needs.
Most general English dictionaries are designed to include only those words that meet certain criteria of usage across wide areas and over extended periods of time (for more details about how words are chosen for dictionary entry, read "How does a word get into a Merriam-Webster dictionary?" in our FAQ). As a result, they may omit words that are still in the process of becoming established, those that are too highly specialized, or those that are so informal that they are rarely documented in professionally edited writing. But the words left out are as real as those that gain entry; the former simply haven't met the criteria for dictionary entry–at least not yet (newer ones may ultimately gain admission to the dictionary's pages if they gain sufficient use).
However, in preparing your own writings, it is worth remembering that the dictionary encompasses the most widely used terms in English. Words that are left out may have usage limited to specific, isolated, or informal contexts, so they should be used carefully.