Part-year resident return
The return you file with a state where you lived for part of the tax year, covering the income you received while you were its resident.
Moving between states mid-year normally produces two of these, one per state, each covering its own months, with deductions, exemptions and credits prorated to the residency period. Because neither state taxes the whole year, part-year filing does not double-tax you by default.
The dividing line is the date your domicile actually changed, which is a question of fact rather than of paperwork: where you live, where your family is, where your possessions are, where you are registered to vote. Keep evidence — a lease, a closing statement, a driving licence issue date.
Two things reliably go wrong. Income received near the move date gets assigned to the wrong state: a final bonus, a vesting equity grant, a last commission cheque. And payroll keeps withholding for the old state for months after the move, which produces a refund on one return and a balance due on the other.
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In one sentence
What is part-year resident return?
The return you file with a state where you lived for part of the tax year, covering the income you received while you were its resident.