A staged map of the user journey, wide at the top (everyone who could hear about you) and narrow at the bottom (the few who actually act), used to spot where users drop off.
Lewis, an American advertising executive, laid out the first formal funnel as Attention, Interest, Desire, Action (AIDA) in 1898 trade publications. The model predates television, the internet, and every digital channel that now uses it, yet AIDA is still taught in marketing 101 classes and embedded in modern CRM dashboards from Salesforce to HubSpot.