The term "market" is used because possible answers to a question are represented as stocks. If you think an answer is correct, you buy stock in it. If you think it is incorrect, you sell. People buying and selling shares makes the stock prices go up and down. Each stock price represents the consensus opinion on what the chances are of that answer being correct. For example, if an answer (stock) has a price of $30, we say that answer has a 30% chance or 3 in 10 chance of happening.