Quote

A pair of prices attached to a size and to a moment. The lower of the two is the bid, the higher is the ask, and the distance between them is the spread. A quote is an indication that the party showing it is willing to deal at those prices, in the size quoted, until it changes, which is a different thing from a trade. A firm quote is dealable as displayed; an indicative quote is a guide to where a market is and carries no obligation behind it.

A retail quote is derived rather than invented. A provider receives two way prices from several liquidity providers, the best bid and the best ask across those sources form the top of the book, and the provider's own charge is applied either inside the quote as a wider spread or beside it as a separate commission, depending on the account's pricing model. The quote is specific to one instrument and one size: the price shown for a small order need not be available for a large one, because the depth behind the top of the book is finite and the remainder of a larger order fills at the next available prices.

The trip is treating a quote as a promise. It describes a moment, and between the moment it is read and the moment an order arrives it can change, which is where slippage, requotes and rejections come from. A chart is not a quote either: most chart lines are drawn from the bid or from the midpoint, so the price a purchase deals at sits above the line by the width of the spread. And the narrower of two quotes is not automatically the cheaper one, because a quote from an account that charges commission separately is narrower than one that carries the same cost inside it, and only the all in figure makes the two comparable.