There are many different kinds of dictionaries on the market--for example, compact, desk-reference, so-called shorter dictionaries, dictionaries of business terms and dictionaries of slang. The full text of the Oxford English Dictionary (begun in the late 1800s, first published 1928; updated since in an attempt to include new words and phrases as they are added to the language) contains over a quarter of a million separate words, and the experts at Oxford estimate that if they were to add up all the distinct meanings and senses for each word, the correct number would be close to three-quarters of a million--either used currently, no longer used or being considered for inclusion.
That's a lot of words. The entries include quotations from published authors illustrating the particular word in context, a system that was first employed by the English lexicographer Samuel Johnson in 1755 and which continues to be a successful method of demonstrating the definition in action.
Before online access was possible, dictionaries were large texts that were difficult to carry and usually ended up propping open doors or windows in college dorms. The need for a standard, comprehensive list of words that could cover all areas of student inquiry, but omit archaic and obsolete entries, was deemed appropriate and desirable.
In the USA, the standard collegiate dictionary is Merriam-Webster's. Most Americans are familiar with the name of Noah Webster, who pioneered a distinctly American listing of language and usage, first published in the U. S. in 1806. This was later revised and expanded by G. & C. Merriam and Company in 1860; since then, Merriam-Webster's has become a standard of excellence. Their current collegiate edition is the eleventh in the series and contains over 225,000 words.
In the UK, the equivalent standard dictionary is Collins (HarperCollins publishers), containing 500,000 words.
As a reference book, the collegiate-level dictionary is an essential tool for any student. Much of the reading assigned outside of the lecture hall or classroom will contain words and phrases not immediately recognizable to anyone not already an expert in the particular field, so having a dictionary open beside the assigned text is to be expected.
A collegiate dictionary should contain any word or phrase that crops up in student research on any topic offered at institutions of higher learning, from astronomy to zoology. In the UK more than in the U. S., however, there is a distinct relationship between vocabulary and class; a university student in the UK has traditionally been expected to possess a wider range of passive and active vocabulary. This is still illustrated in the television quiz show "University Challenge" (first aired 1962, and continuing today), where students are expected to be able to understand and use a wider range of vocabulary (and general knowledge) than their nonuniversity counterparts.
As well as the traditional hard- or paper-backed book, dictionaries are also now available in electronic form (the 2003 second edition of the Oxford Dictionary, for example, is made by Sharp and contains around 355,000 words--as well as a thesaurus and dictionary of quotations--and is quite expensive).
But access is also available online. Most universities, colleges and schools have access to the OED online (you need to be a registered student or faculty member to sign in), which supplies listings for even the most archaic words a student of the language itself could desire. The Merriam-Webster's Collegiate Dictionary is also available to subscribers.