A security that is so similar to another that the Internal Revenue Service (IRS) does not recognize a difference between the two. Substantially identical securities can include both new and old securities issued by a corporation that has undergone reorganization. Convertible securities and common stock of the same corporation can also fall into this category if the market and conversion prices are similar.

Substantially identical securities cannot be used in tax swaps to create capital losses. Securities used in swaps must be similar but not substantially identical; if they are, the IRS will consider the transaction to be a wash sale, and will disallow it.