Leads generated by you reaching out to them first. The seller initiates.
Outbound is cold outreach: cold emails, cold calls, LinkedIn DMs, direct mail. You target prospects who haven't asked to hear from you, but fit your ICP. Done well, it's a fast, controllable way to generate pipeline. Done poorly, it's spam that damages your brand. SDRs are often the outbound engine.
Outbound is how early-stage companies generate pipeline before they have content or brand. It's also how enterprise companies reach specific named accounts. Know when to use it: accounts too high-value to wait for inbound, ICP that doesn't hang out online, or you just need pipeline NOW.