A trade hold is a period of time after a trade is accepted by both parties where the items traded are held by Steam before they can be delivered. Trade holds can last up to 15 days. If you're trading items with a friend, and you've been friends for more than 1 year, the hold is 1 day.

If you haven't had your account protected by a Mobile Authenticator for at least the last 7 days, items leaving your account will be held by Steam for up to 15 days. This provides you time to discover any transactions on hold that you didn't make. If you find that your account has been compromised, you can change your Steam account password and cancel any item transactions that are pending before your items are stolen.
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When you cancel a trade that is being held by Steam, Steam will assume that you intercepted a hijacker and you'll see a 7-day trading cooldown to prevent any further unauthorized attempts to trade away items. This cooldown is for your protection and cannot be removed by Steam Support.

Trade and market holds are no longer necessary when you protect your account with a Steam Guard Mobile Authenticator. Using a physical device (your smartphone) to confirm item transactions ensures that you and only you can quickly and securely trade or sell your items. You can get the Steam Guard Mobile Authenticator for iOS, Android, and Windows Phone devices. Items will no longer be held after you have had the authenticator enabled for 7 days.

It is possible to switch Mobile Authenticator between phones and avoid 15 days trade ban. Here is a solution to shorten this time to only 2 days:

That’s it. Sign in and enjoy a shorter tradehold period 😎
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​Thanks to StrozeR for the reduced trade hold guide.
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