Definition
Open-Source Software
Software whose source code is publicly viewable, so independent parties can verify what it actually does rather than trusting the vendor's claims.
Open-source software publishes its code so security researchers, auditors, and any curious developer can inspect exactly how it handles your data. This does not automatically make it secure, but it makes claims falsifiable — a closed-source "we don't log anything" is a promise; an audited open-source equivalent is closer to a fact.
Why it matters: when evaluating privacy tools, open-source plus independent audit is the strongest verification available to an ordinary user who cannot personally read the code.
Related terms
Zero-Knowledge · VPN (Virtual Private Network) · Password Manager