An Accounting Period is a company's financial year i.e. the period for which it prepares statutory accounts.
Under the Companies Act 2006 an accounting (reference) period runs from one accounting reference date to the next. A company's first accounting period is a period of 6 to 18 months starting on the date of incorporation and ending on the company's accounting reference date. Subsequent accounting periods are 12 months long starting immediately after the end of the previous accounting period and ending on the accounting reference date.