Wash Sale
Selling something at a loss and buying it back almost immediately, which some tax systems treat as a loss you may not claim.
The United States wash-sale rule in section 1091 of the Internal Revenue Code disallows a loss when substantially identical stock or securities are reacquired within a window around the sale, with the disallowed loss added to the basis of the replacement position rather than lost outright. Whether the rule reaches digital assets depends on whether they are treated as securities for this purpose, which is why the point has been the subject of repeated legislative proposals rather than a settled answer. Other countries address the same behavior through different mechanisms, such as same-day and short-window matching rules. A separate term is routinely confused with this one: wash trading means buying and selling with yourself to fabricate volume, which is market manipulation rather than a tax matter. Rules vary by jurisdiction and change; this describes the concept and is not tax advice.
In practice
Where the rule applies, the disallowed loss is not destroyed but added to the cost basis of the repurchased position, deferring the effect rather than removing it.
The common misunderstanding
That a wash sale and wash trading are the same thing, when one is a tax rule about repurchasing a position sold at a loss and the other is manipulation that fabricates trading volume.