function

1. General: Type of action performed by a device, department, or person. Function remains more or less fixed whereas the 'purpose' (which indicates intention or objective) generally changes. For example, the function of a hammer is to strike something nearby whereas its purpose (what to strike and why) could be anything the hammer-wielder has in mind.

2. Management: Task oriented block of related efforts (accounting, marketing, manufacturing, etc.) organized to produce intended outputs (financial reports, sales, products, etc.).

3. Statistics: Mathematical relationship in which a quantity (dependent variable) depends on or is determined by another quantity (independent variable) or quantities. The dependent variable is said to be a function of the independent variable(s). If something is done, or something happens, to the independent variable(s), it is reflected in the dependent variable. For example, expenditure is a function of income and, for a wage earner, income is a function of two variables-per hour wage rate and number of hours worked.

Cofounders are for a startup what location is for real estate. You can change anything about a house except where it is. In a startup you can change your idea easily, but changing your cofounders is hard. And the success of a startup is almost always a function of its founders.