Effectively Connected Income (ECI) Law and Legal Definition
Non-resident alien individuals and foreign corporations engaged in trade or business within the US are subject to US income tax on income, from sources both within and outside the US, which is "effectively connected" with the conduct of the trade or business within the US. Income is effectively connected if it is derived from assets which are used in or held for use in the US, and the activities of the US business were a material factor in the realization of the income.
Generally, when a foreign person engages in a trade or business in the United States, all income from sources within the United States connected with the conduct of that trade or business is considered to be Effectively Connected Income (ECI). This applies whether or not there is any connection between the income, and the trade or business being carried on in the United States, during the tax year. It is taxed at the graduated rates or lesser rate under a tax treaty.