Look-back period
In short
How far back an audit reaches. Four years is the normal reach, and normal is not the same as maximum.
In full
The look-back period is the span of past periods an audit actually examines. ADOR states that an audit normally covers the most recent four-year period.
Normal is a practice, not a limit. What sets the outer boundary is the statute of limitations, and where no return was filed for a period, A.R.S. § 42-1104(B)(1)(b) sets no boundary at all.
The phrase carries a second and more useful meaning in the Voluntary Disclosure Program, where a limited look-back period is the benefit being offered rather than a description of an auditor's habit. There the standard period is four years from the date the application is submitted, and the agreement includes a waiver of the Department's ability to audit periods before it.
The two senses point in opposite directions. In an audit the look-back is what the Department chooses to examine and can extend. In a voluntary disclosure it is what the Department agrees to give up.
For a contractor several years behind, that difference is the whole decision.
A.R.S. § 42-1104(A), (B)(1)(b); ADOR TPT Audit; ADOR Voluntary Disclosure and Compliance Program