Effective gross income (EGI) is the revenue line you can actually count on. Start with gross potential rent — every unit at market rent, fully occupied, all year — subtract vacancy loss and credit loss (rent you never collect from non-payment), then add other income: pet fees, parking, laundry, storage, application fees. What remains is the income your operating expenses come out of.
EGI sits between two more famous numbers. It is the honest version of gross potential rent, and it is the top line from which operating expenses are subtracted to reach net operating income. When you underwrite a deal from a seller's pro forma, EGI is where the optimism usually hides: a vacancy allowance that is too thin, or "other income" that the property has never actually produced. Rebuild it from the actual rent roll and trailing collections, not the brochure.