Your Word Score is an estimate of how many words you know. This measure combines innovative scholarly research on how vocabulary works along with a proprietary algorithm that assigns a difficulty value to individual words.
Your Word Score grows when you master new words. We consider a number of variables, including the difficulty value of the word, dependence on other words for comprehension, what words you have already mastered, and more to determine how much to add to your score. The behavior of this algorithm (along with the general character of the English language) means that your Word Score may increase by a lot, or may increase by just a little, depending on the word you've learned.