A word or phrase people type into Google. The atomic unit of SEO.
Keywords are what users search for. Every SEO strategy starts with keyword research: what are potential customers typing, how many times per month, how competitive is each one? Keywords have 'intent' (informational, navigational, transactional, commercial), and the intent determines what kind of content ranks. 'How to fix a leaky faucet' wants a how-to. '[brand] review' wants a review.
Picking the wrong keywords is the #1 SEO failure mode. High-volume keywords are competitive; low-volume keywords don't drive traffic. The sweet spot is medium-volume keywords where you can realistically rank and where the users actually want what you sell.