An exit tax treats your departure as if you sold your assets the day you leave, taxing the paper gains even though you haven't actually sold. The US imposes one on certain 'covered expatriates' who renounce citizenship or give up long-term residency; Canada, Australia, and several European countries have their own versions tied to ceasing residency.
Thresholds, asset types, and exemptions vary widely, and the tax can be significant for people with appreciated investments or business stakes. Planning the timing and structure of a departure can materially change the bill.
Why it matters for your move
If you're leaving a country for good — especially renouncing US citizenship — an exit tax can be the largest single cost of the move. It's the kind of thing to model with a professional well before you go.