183 days is roughly half a year, and it's the most widespread trigger for tax residency. Cross it in most countries and you're presumed to be a tax resident, with all the obligations that follow — though the exact day-count rules (calendar year vs rolling 12 months, how partial days count) vary.
It's a presumption, not the whole test. Some countries make you resident on fewer days if your main home or family is there; others have day-count tie-breakers in their tax treaties. Counting days carefully — and keeping records — matters if you're splitting the year across places.
Why it matters for your move
If you're trying to stay tax-resident somewhere favourable (or avoid becoming resident somewhere costly), the 183-day line is the one you plan your calendar around.