A loss is associated with a "peripheral" or "incidental" transaction. Examples of losses include the loss on the sale of an asset used in the business, loss from a lawsuit settlement, and loss from retirement of bonds. However, there are some losses that are closer to operations, such as the loss on write-down of inventory from cost to market.
The FASB's Statement of Financial Accounting Concepts No. 6 (December 1985) discusses expenses and losses in paragraphs 80-89. You can access this concepts statement at no cost at www.FASB.org/st.