Definition
If you sell at a loss and you or an affiliated person acquires the same or an identical property in the period starting 30 days before and ending 30 days after the sale, and still holds it at the end, the loss is denied for the time being and added to the new cost. It is the Canadian counterpart of the US wash sale rule.
Watch out for
It also applies across accounts and to some family members, including inside registered accounts. Check CRA guidance.