What GAAP principle states that all expenses incurred while earning revenue should be reported in the same period that the income is reported?

The GAAP principle that states all expenses incurred while earning revenue should be reported in the same period as the income is known as the "Matching Principle." This principle ensures that expenses are matched with the revenues they help generate, providing a more accurate representation of a company's financial performance during a specific period. This alignment helps stakeholders understand the true profitability of the business.