The practice of turning strangers into people who've given you permission to market to them.
Lead gen is the front door of direct response. You run ads, write content, send cold emails, whatever, all with one goal: get strangers to raise their hand and say 'tell me more.' Once they're leads (email, phone, form submission), you can market to them over time. Lead gen is about volume and cost, not immediate sales.
Most B2B and higher-ticket businesses run on lead gen + nurture. If you're not capturing leads, you're renting attention from ads and then throwing it away. Build a real lead gen engine and your marketing economics improve forever.