Merriam-Webster's abridged dictionaries, such as Merriam-Webster's Collegiate Dictionary, include a lexicon of the more common words used at home, in school, or in the office, but exclude most words of low or infrequent usage and words that may be used in specialized contexts. This abridgement is necessary to get the perfect size for an everyday, desktop reference book. But exclusion from our abridged dictionaries does not mean the word is not in any Merriam-Webster dictionary. Webster's Third New International Dictionary, Unabridged includes many of the words not included in the more compact and concise abridged editions.
All of the words entered in Merriam-Webster dictionaries are selected on the basis of actual evidence of usage; how much usage is required to gain entry into the dictionary depends on many factors including the size of the finished dictionary. Before any word can be considered for inclusion, we have to have proof not only that it has existed in the language for a number of years, but also that it is frequently used by all sorts of English writers (as indicated by how often it appears in a variety of general-interest publications). Words that turn up relatively infrequently or only in very specialized contexts may not be candidates for entry in an abridged dictionary, but might well be included in the unabridged dictionary. For a more detailed explanation of how words are chosen for inclusion in the dictionary, see the article "How does a word get into a Merriam-Webster dictionary?".
People often ask us why they can't find certain words in their dictionary, and the answer usually is that they just need to check a larger dictionary. However, it is worth noting that words are sometimes missing from the dictionary because they are too new, too specialized, "nonlexical", or otherwise inappropriate for entry into the dictionary. See the article "If a word is not in the dictionary, does that mean it isn't a real word?" for more information.