Capital Loss
A capital loss is what you have when you sell an investment for less than you paid. It isn't only bad news: losses offset capital gains dollar-for-dollar, and up to $3,000 of net loss can offset ordinary income each year, with the rest carried forward to future years.
Deliberately realizing losses to reduce taxes is called tax-loss harvesting. Losses only count when you actually sell, and the wash-sale rule blocks the deduction if you rebuy the same security within 30 days.