Market Order

A market order executes immediately at the best available price, consuming the order book level by level until the full size is filled. It guarantees the trade happens; it says nothing about the price you get.

In liquid large caps the cost is usually a spread's worth of cents. In thin names or fast tape, a market order can walk several levels deep and fill far from the quote you saw when you clicked. The rule of thumb: market orders buy certainty of execution, and the thinner the book, the more that certainty costs.