How many words are there in the English language?

There is no single sensible answer to this question.

It is impossible to count the number of words in a language, because it is so hard to decide what counts as a word. Is dog one word, or two (a noun meaning 'a kind of animal', and a verb meaning 'to follow persistently')? If we count it as two, then do we count inflections separately too (dogs plural noun, dogs present tense of the verb). Is dog-tired a word, or just two other words joined together? Is hot dog really two words, since we might also find hot-dog or even hotdog?

It is also difficult to decide what counts as 'English'. What about medical and scientific terms? Latin words used in law, French words used in cooking, German words used in academic writing, Japanese words used in martial arts? Do you count Scots dialect? Youth slang? Computing jargon?

The Second Edition of the Oxford English Dictionary contains full entries for 171,476 words in current use, and 47,156 obsolete words. To this may be added around 9,500 derivative words included as subentries. Over half of these words are nouns, about a quarter adjectives, and about a seventh verbs; the rest is made up of interjections, conjunctions, prepositions, suffixes, etc. These figures take no account of entries with senses for different parts of speech (such as noun and adjective).

This suggests that there are, at the very least, a quarter of a million distinct English words, excluding inflections, and words from technical and regional vocabulary not covered by the OED, or words not yet added to the published dictionary, of which perhaps 20 per cent are no longer in current use. If distinct senses were counted, the total would probably approach three quarters of a million.

--Reference: www.askoxford.com