What is Fiscal Year?

Legal Definition
A fiscal year (or financial year, or sometimes budget year) is the period used by governments for accounting and budget purposes, which vary between countries. It is also used for financial reporting by business and other organizations. Laws in many jurisdictions require company financial reports to be prepared and published on a generally annual basis, but generally do not require that the reporting period be calendar year, 1 January to 31 December. Taxation laws generally require accounting records to be maintained and taxes calculated on an annual basis, which usually corresponds to the fiscal year used for government purposes. The calculation of tax on an annual basis is especially relevant for direct taxation, such as income tax. Many annual government fees—such as Council rates, licence fees, etc.—are also levied on a fiscal year basis, while others are charged on an anniversary basis.

The 'fiscal year end' (FYE) is the date that marks the end of the fiscal year. Some companies—such as Cisco Systems—end their fiscal year on the same day of the week each year, e.g. the day that is closest to a particular date (for example, the Friday closest to 31 December). Under such a system, some fiscal years will have 52 weeks and others 53 weeks.

The calendar year is used as the fiscal year by about 65% of publicly traded companies in the United States and for a majority of large corporations in the UK and elsewhere, with notable exceptions being in Australia, New Zealand and Japan.

Many universities have a fiscal year which ends during the summer, both to align the fiscal year with the academic year (and, in some cases involving public universities, with the state government's fiscal year), and because the school is normally less busy during the summer months. In the northern hemisphere this is July to the next June. In the southern hemisphere this is calendar year, January to December. Some media/communication based organizations use a broadcast calendar as the basis for their fiscal year. The American football league NFL uses the term "league year," which in effect forms the league's fiscal year.

The fiscal year is usually denoted by the year in which it ends, so United States of America federal government spending incurred on 14 November 2017 would belong to fiscal year 2018, operating on a fiscal calendar of October–September.
-- Wikipedia
Legal Definition
In the administration of a state or government or of a corporation, the fiscal year is a period of twelve months (not necessarily concurrent with the calendar year) with reference to which its appropriations are made and expenditures authorized, and at the end of which its accounts are made up and the books balanced. See Moose v. State, 49 Ark. 499, 5 S. W. 885.
-- Black's Law Dictionary