What is defined as a taxpayer's legal, permanent residence?

The correct answer is defined as a taxpayer's legal, permanent residence because "domicile" specifically refers to the place where a person has their fixed, permanent home and where they intend to return. It encompasses not just the physical house in which a person may live, but also the broader concept of being legally tied to that location for personal, legal, or tax purposes.

While "residency" can refer to where someone lives at a given time, it does not always imply permanence or intent to remain there. "Permanent address" often indicates a mailing location but might not capture the legal implications of domicile. "Home base" is a more colloquial term that does not have the same legal significance as domicile concerning tax and residency laws. Thus, "domicile" provides the most accurate definition in the context of legal residence for tax purposes.