In accounting, periodicity means that accountants will assume that a company's complex and ongoing activities can be divided up and reported in annual, quarterly and monthly financial statements. For example, some earth-moving equipment may require two years to manufacture but the activities will be divided up and reported in quarterly financial statements. A similar situation occurs at a company that develops complex digital systems.
Even a company that manufactures small consumer products will have ongoing activities and costs that overlap two years or more. Again, the accountants will assume that the revenues and costs can be assigned or allocated to the appropriate accounting periods. Hence, the accountants will report the company's net income and cash flows for each accounting period (year, quarter, month, etc.) and the company's financial position at the end of each accounting period.
Periodicity is also known as the time period assumption.