Depending on context, the term may also refer to listed company (quarterly) earnings guidance.
Typically, using historical internal accounting and sales data, in addition to external industry data and economic indicators, a financial forecast will be the analyst's modeled prediction of company outcomes in financial terms over a given time period.
Arguably, the key aspect of preparing a financial forecast is predicting revenue; future costs, fixed and variable, as well as capital, can then be estimated as a function of sales via "common-sized analysis" - where relationships are derived from historical financial ratios and other accounting relationships.
[1] At the same time, the resultant line items must talk to the business' operations:- in general, growth in revenue will require corresponding increases in working capital, fixed assets (see, here, owner earnings) and associated financing; and in the long term, profitability (and other financial ratios) should tend to the industry average;[2] see Valuation using discounted cash flows § Determine cash flow for each forecast period for more detailed discussion, and other considerations; also Cash flow forecasting.
There is an extensive literature on the accuracy of analyst forecasts of revenue, profit and share price developments of companies.