Tax residency refers to the determination of an individuals tax status in a particular jurisdiction. The criteria for tax residency vary from country to country, and residency can be different for non-tax purposes. In general, an individuals physical presence in a jurisdiction is the main test for determining tax residency. However, some jurisdictions also consider other factors such as the ownership of a home, availability of accommodation, family, and financial interests. For companies, some jurisdictions determine residency based on the place of incorporation.

In the United States, all U.S. citizens and U.S. residents are treated as U.S. tax residents. Non-U.S. citizens can be treated as resident aliens if they satisfy either the "green card test" or the substantial presence test. The green card test applies to individuals who were legal permanent residents of the United States at any time during the past calendar year, while the substantial presence test applies to individuals who were physically present in the United States for at least 31 days during the current year and 183 days during the 3-year period that includes the current year and the 2 years immediately before that.

It is important to note that tax residency is determined under the domestic tax laws of each jurisdiction, and there might be situations where a person qualifies as a tax resident in more than one jurisdiction. Therefore, it is essential to understand the tax residency rules of each jurisdiction where an individual has a tax obligation.