What is loss? What is loss? The act or an instance of losing: nine losses during the football season. One that is lost: wrote their flooded house off as a loss. The condition of being deprived or bereaved of something or someone: mourning their loss. The amount of something lost: selling at a 50 percent loss. The harm or suffering caused by losing or being lost.People lost in wartime; casualties.Destruction: The war caused incalculable loss. Electricity The power decrease caused by resistance in a circuit, circuit element, or device.The amount of a claim on an insurer by an insured.at a loss Below cost: sold the merchandise at a loss. at a loss Perplexed; puzzled: I am at a loss to understand those remarks.