ABM is essentially a form of strategic business marketing. An organization takes an individual prospect or customer account -- these are companies, not individual people -- and treats it like its very own market, or a market of one.
Account-based marketing is a strategic marketing strategy where key business accounts are marketed to directly, as units of one (compared to the typical one-to-many approach). In essence, high-value accounts or prospects are identified, key stakeholders in these businesses are targeted, and then marketing strategies are implemented through various channels to appeal to their specific personas and needs. Account-based marketing is like personalized marketing on steroids.