A document detailing a company's assets and liabilities.
The two quantities that have to "balance" or equal each other are (1) assets and (2) the sum of liabilities and shareholders' equity;
ie the total amount of assets the company controls is equal to what they own plus what they've borrowed.
The balance sheet is interesting to stock investors because it provides details on the actual assets that make up a stock's book value.
See the main article for a sample balance sheet.