There is no definitive number of words in the English language although Webster's Third New International Unabridged Dictionary contains approximately 470,000 entries. The difficulty in quantifying the number of words in English results from the constantly evolving use of the language.
The problem of quantifying the number of words in English also arises from the tendency of one word, such as "post," to have several different meanings. English also makes extensive use of words with foreign origins, such as “espresso” or “shiatsu,” not to mention a myriad of academic, legal, technical, scientific and medical terms from Greek, Latin and other languages. There is also a question of including items such as “bill of lading” or “a la carte” that have distinct meanings which are not possible to determine from the meanings of their component words. There is also no way to determine what constitutes slang or obsolete words.