In general, a fiscal year is a period of time (typically 12 months) used for accounting and budgeting. While a "calendar year" that runs from January to December is often used by individuals, companies, and organizations for financial and tax reporting, other organizations use a different period of twelve months.
In the context of Wikimedia, the fiscal year is used for the annual plan, the budget, and financial reports, including reports filed with U.S. tax agencies. The Wikimedia Foundation fiscal year runs from July 1 to June 30 of the following year. For example, Fiscal Year 2018–19 will run from July 1, 2018 to June 30, 2019.
Wikimedia affiliates are separate legal entities and may, or may not, use the same fiscal year as the Foundation. The Wiki Project Med Foundation uses the calendar year, while Wikimedia District of Columbia uses October 1 to September 30 as its fiscal year.