This includes all income you receive in the form of money, goods, property, and services that is not exempt from tax. It also includes income from sources outside the United States (even if you can exclude all or part of it).
Include part of your social security benefits if:
If either (1) or (2) applies, see the instructions for Form 1040 or 1040A, or Publication 915, Social Security and Equivalent Railroad Retirement Benefits, to figure the social security benefits you must include in gross income. If you are married filing separately and you lived in a community property state, then you must include your portion of any community income.
This information is found in the Filing Information chapter of Publication 17, Your Federal Income Tax.