Most of the time a submitted buy or sell order in the stock market can be canceled. It could be rejected or canceled by a brokerage firm if, for example, you placed a buy stop order that is below the current market price, or a sell stop order price has already been reached. Or perhaps you placed an order with a broker that does not handle the stock you are interested in buying.
However, any market order for let's say 200 shares of Never Cancel Inc. is executed immediately, so it would be next to impossible to cancel. But a limit order such as "buy 100 shares of Never Cancel Inc. when it hits $20 a share," should give you time to cancel either online or through a broker.
There is also such a thing as "Fill or Kill" orders. Let's say you wanted to buy 500 shares of a stock but only 400 shares are available at the price you wanted. Since the entire order can't be completed, it will be canceled. To add to the list, there are one-cancels-the-other orders (OCO). Johnny Trader thinks Take a Chance On Me Inc. might break out and go up above the current price range of $25 to $50, but also wants to protect himself on the off chance he is wrong. So he places an OCO with a buy limit of $52 and a sell limit for $23. If the stock hits $52 the buy order goes through and the sell order is canceled. And alternatively, if it goes down to $23 the sell order is executed and the buy order canceled.
Related or Semi-related Video
Finance: What is a market order?3 Views
Finance a la shmoop what is a market order? alright people it's a type of buy
or sell order which simply says I will buy or sell this stock or bond at this
price right now that is whatever price the market is telling me it's worth [Investor discussing market orders]
right now that's my price so the market prices change all the time and yes of
course the broker can choose to look at his screen this minute or that minute or [Broker looking at computer screen]
the following minute and the market will be changing if you're placing the order
during the day so yes there is some discretion in defining exactly what
dollar amounts and penny amounts a market order means but in real life
usually through online portals or even through old-school telephones yes those [Person using an old-style telephone]
things the broker will say something like yep I see coca-cola quota here at
42.48 that's market right now you want it and if you want to sell well then you [Woman selling bonds while on the phone]
mumble something like go ahead and just so you know the kissing cousin of a
market order is a limit order like an investor might tell the broker give me a
42.50 limit on that coke sale if the stock never hits 42.50 like it's
below at 42.49 doesn't do it well then no sale ever happens and the [Coca cola stock price graph stamped with no sale]
owner of those coca-cola shares well they just keep them if the stock
suddenly pops and it gets there then blam the shares have been sold well in
practice most relatively sophisticated buyers and sellers put in limits like
limit orders not just naked market orders even if they are below what the
market currently floats because flash crashes happen all the time it seems
these days and in one fleeting moment shares of coca-cola could suddenly [Coca cola share price drops]
plummet to 34.12 a share and if the seller has a market order in while those
shares will transact or be sold at 34.12 even though 15 minutes later the markets
back to its 42.50 a share pricing so you can get really heard if you rely on
the market being a stable and rational so vanilla pure market orders can be a [Man with pigs head riding a bike]
dangerous thing be careful just ask the little piggy you know who went to market
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