In one sentence
A property's revenue minus operating expenses, before debt service, capital items, and income taxes.
The longer version
NOI is the number underneath every sizing metric: DSCR, debt yield, and value via the cap rate. Rental and other income, less vacancy, less operating expenses like taxes, insurance, utilities, repairs, and management.
The arguments live in the definitions. In-place versus stabilized, actual versus underwritten expenses, and whether reserves count as operating all change the number, so loan documents and appraisals spell out the recipe they use.
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