An unrealized loss is a decline in the value of an asset that has not yet been sold. One might continue to hold such an asset in the expectation that it will gain in value, perhaps offsetting the amount of the current unrealized loss.
When an asset is sold, it becomes a realized loss. Only a realized loss can be used to offset a taxable gain for the purpose of reducing one's income tax liability.
For example, ABC Company owns an investment that cost $100,000, but which now has a market value of $80,000. ABC therefore has an unrealized loss of $20,000.
An unrealized loss is also known as a paper loss.