Vocabulary and Intelligence:

>[The] concepts represented by some words are too complex, abstract, or subtle for some people to infer from any context or to fully understand even when the word is fully defined. A person may look up the definition in the dictionary and might even memorize it verbatim; but unless the meaning of the word is grasped at a deeper, nonverbal conceptual level, it does not become a part of his functional vocabulary. It is remarkable how hard it is to retain such words - the memorized definition soon fades beyond retrieval. And even if the memorized definition is provided again, the person's lack of conceptual grasp of the word's meaning is shown by his inability to express the meaning adequately in words other than those of the memorized definition. There is a very high correlation between the subtlety with which people understand the meaning of words, and the sheer number of different words whose meaning they can recognize in any sense. Thus, vocabulary or word knowledge is a good indicator of g, provided, of course, that the test words are not too narrowly selected from specialized areas of learning.<

Arthur Jensen, Straight Talk About Mental Tests, 1981