A business plan is a written document that sets out what a business does, who it sells to, how it makes money, and what resources it needs to reach its goals. It combines a written narrative with a financial model, and it exists so that a reader — a lender, an investor, a partner, or you — can judge whether the business is viable.
A business plan answers four questions in order: what problem does this business solve, who pays to have it solved, how does the business deliver and profit from solving it, and what has to be true for the numbers to work. Everything else in the document is supporting evidence for those four answers.
The distinguishing feature of a plan — versus a pitch deck, a strategy memo or a one-page canvas — is that it carries a financial model. Projected revenue, costs, profit and cash flow are stated as figures a reader can check against the narrative.
A traditional business plan runs roughly 15 to 40 pages and is what banks, SBA lenders, grant committees and immigration reviewers expect. A lean or one-page plan compresses the same logic into a single page of assumptions and works as an internal thinking tool rather than a fundraising document.
Write the financial model first, or at least in parallel. Most weak plans are written narrative-first and have numbers attached at the end, which is why their sections contradict each other. Finish with the executive summary — it is the first page but the last thing that can honestly be written.