What is the greek word meaning total stock of words in a language?

There isn't one single Greek word that perfectly translates to "total stock of words in a language" (lexicon or vocabulary). However, λεξικόν (lexikon) comes closest. While often used to refer to a dictionary, its root λέξις (lexis) means "word," and thus *lexikon* can conceptually encompass the entire collection of words in a language.