Cost basis is the amount treated as your acquisition cost for a crypto asset — usually what you paid (including certain fees), or the fair market value when you received it (for example via mining, staking, or an airdrop), depending on local rules.
When you later dispose of the asset, taxable gain/loss is typically disposal proceeds minus cost basis (subject to jurisdiction-specific adjustments).
Why it matters: without a reliable cost basis, capital gains calculations and software reports are wrong.
Educational definition — not tax advice. Rules vary by country.
