A precise description of the company or person you most want as a customer.
The ICP is who you sell to, ideally. For B2B: industry, company size, role of the buyer, tech stack, signals that suggest they need you. For B2C: demographics, psychographics, behaviors, life stage. A good ICP is specific enough that you can look at any account and say 'in ICP' or 'out of ICP.' A vague ICP is a sign you haven't figured out who you sell to.
Every hour spent marketing to non-ICP customers is wasted. Narrowing your ICP feels scary ('we'll miss people!'), but it focuses your spend, sharpens your messaging, and shortens your sales cycle. Bigger target isn't better; more-right target is better.