In financial and business accounting, earnings before interest and taxes (EBIT) or operating income is a measure of a firm's profitability that excludes interest and income tax expenses.Bodie, Z., Kane, A. and Marcus, A. J. Essentials of Investments, McGraw Hill Irwin, 2004, p. 452. ISBN 0 07 251077 3
EBIT = Operating Revenue – Operating Expenses (OPEX) + Non-operating Income

Operating Income = Operating Revenue – Operating Expenses
Operating income is the difference between operating revenues and operating expenses, but it is also sometimes used as a synonym for EBIT and operating profit. This is true if the firm has no non-operating income.

A professional investor contemplating a change to the capital structure of a firm (e.g., through a leveraged buyout) first evaluates a firm's fundamental earnings potential (reflected by Earnings Before Interest, Taxes, Depreciation and Amortization EBITDA and EBIT), and then determines the optimal use of debt vs. equity.

To calculate EBIT, expenses (e.g., the cost of goods sold, selling and administrative expenses) are subtracted from revenues. Profit is later obtained by subtracting interest and taxes from the result.

(Table info source: Bodie, Z., Kane, A. and Marcus, A. J. Essentials of Investments, McGraw Hill Irwin, 2004, p. 452.)