The General Explanations at the beginning of the Oxford English Dictionary states:

The Vocabulary of a widely diffused and highly cultivated living language is not a fixed quantity circumscribed by definite limits... there is absolutely no defining line in any direction: the circle of the English language has a well-defined centre but no discernible circumference.

The current FAQ for the OED further states:

How many words are there in the English language? There is no single sensible answer to this question. It's impossible to count the number of words in a language, because it's so hard to decide what actually counts as a word.

The vocabulary of English is undoubtedly vast, but assigning a specific number to its size is more a matter of definition than of calculation. Unlike other languages such as French (theAcadémie française), German (Rat für deutsche Rechtschreibung), Spanish (Real Academia Española) and Italian (Accademia della Crusca), there is no academy to define officially accepted words and spellings. Neologisms are coined regularly in medicine, science, technology and other fields, and new slang is constantly developed. Some of these new words enter wide usage; others remain restricted to small circles. Foreign words used in immigrant communities often make their way into wider English usage. Archaic, dialectal, and regional words might or might not be widely considered as "English".

The Oxford English Dictionary, 2nd edition (OED2) includes over 600,000 definitions, following a rather inclusive policy:

It embraces not only the standard language of literature and conversation, whether current at the moment, or obsolete, or archaic, but also the main technical vocabulary, and a large measure of dialectal usage and slang (Supplement to the OED, 1933).

The editors of Webster's Third New International Dictionary, Unabridged (475,000 main headwords) in their preface, estimate the number to be much higher. It is estimated that about 25,000 words are added to the language each year.

The Global Language Monitor announced that the English language had crossed the 1,000,000-word threshold on June 10, 2009. The announcement was met with strong scepticism by linguists and lexicographers, though a number of non-specialist reports accepted the figure uncritically.

Comparisons of the vocabulary size of English to that of other languages are generally not taken very seriously by linguists and lexicographers. Besides the fact that dictionaries will vary in their policies for including and counting entries, what is meant by a given language and what counts as a word do not have simple definitions. Also, a definition of word that works for one language may not work well in another, with differences in morphology and orthography making cross-linguistic definitions and word-counting difficult, and potentially giving very different results. Linguist Geoffrey K. Pullum has gone so far as to compare concerns over vocabulary size (and the notion that a supposedly larger lexicon leads to "greater richness and precision") to an obsession with penis length.