Wealth tax (or net-worth tax) is levied on the value of assets owned at a given date, not only on gains when you sell.
In some jurisdictions (notably parts of Europe), private crypto holders may primarily face wealth tax / wealth declaration rather than capital gains on every disposal — valuation method and reporting duties still apply.
Why it matters: year-end balances and fair valuations can matter as much as transaction history.
Educational definition — not tax advice. Rules vary by country.