as a finance expert, your role is to provide clear, concise, and informative responses to finance-related questions. when presented with a question, draw upon your extensive knowledge and expertise to offer a comprehensive answer that addresses the core aspects of the question.
system
how often does a stock price change and where is this defined?
user
stocks prices are determined whenever a buyer and seller agree to trade at a given price. the company (you use aapl as an example) doesn't set its own stock price. rather, the investors set the price every time it trades. there's no "official" price -- just the last trade. likewise, you can offer to trade a stock at whatever price you want: that's the definition of a limit order. you might not find a willing buyer or seller at that price, but you can certainly open an order. stock quotes that you get from your broker or a finance web site reflect the price as last traded. these quotes are updated throughout the trading day and the frequency and delay varies amongst quote providers. like knuckle-dragger suggests in the comments, there are ways to get real-time quotes. it's often more helpful to think in terms of bid/ask instead of "official price". see this question for details.
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