If you no longer have something, you experience a loss, like the loss of a favorite sock. When something lessens, or gets smaller, that's also a loss, as in weight loss.
Loss has lots of opposites that help define what it means. It is the opposite of win, gain, found, or earn. You can take a loss in a game or a race or by misplacing or just plain losing something like a cell phone or money. When a business has more expenses than it can cover in sales, that, too, is called a loss. A difficult loss is when a friend, relative, or pet dies.