A decision-making unit, or DMU, is the set of people inside a company who influence, approve or block a purchase. It typically includes the economic buyer who controls money, a champion who advocates for you, technical reviewers who check criteria, and the users who will live with the product. Few sales are decided by one person; most are decided by a committee. Mapping the DMU early is how you avoid losing to a veto you never met.
In plain English
Think of a deal as a small election. You need votes from the person who signs, the person who defends you, and the person who can say no on technical grounds. Ask "who else is involved in this decision and what does each of them care about?" and you have your map.
Why it matters to you
A deal with one strong contact is fragile; a deal with the whole unit on board is real. Reps who map the DMU qualify better, tailor each conversation to each person, and stop losing deals to "I loved it but security said no." It is a core skill in enterprise sales.
For qualifying the decision process, timeline and authority as an AE, see Qualify the Decision Process.