A service that pools cryptocurrency transactions from multiple users to obscure the trail between sender and receiver. You send coins in; they get shuffled with others' coins; different coins come out the other end. The goal: break the on-chain link between your input and output addresses.
Mixers work because blockchain is transparent by default. Every transaction is recorded publicly. A mixer introduces plausible deniability—your output address can't be reliably linked to your input. Some use coinjoin protocols (multiple parties signing one transaction); others use centralized pools or tumblers.
Why it matters: Privacy on public blockchains is not automatic. If you care about transaction privacy—whether for legitimate financial privacy, jurisdictional arbitrage, or circumventing surveillance—a mixer is one tool. It's not magic; forensic analysis can sometimes pierce mixing, especially if you're sloppy about wallet hygiene.
Legal status varies wildly. Most jurisdictions don't ban mixing itself, but use of mixers can trigger AML/KYC scrutiny from exchanges. Some exchanges now refuse to accept mixer-sourced coins. This is an arms race, not a settled question.