Most systems that export resources, either directly or otherwise, come with some implicit understanding that those resources will generally be accessible (available). If an availability problem is caused maliciously, it is known as a denial of service attack.
Note that data delays can be considered an availability problem. For example, imagine sending a message that says, “sell 10 shares of MSFT” that an attacker delays until the price has plummeted to the point where the user would no longer want to sell those shares.
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