Source: Image of gears, public domain, http://www.clker.com/clipart-2950.html, All graphs by Justin Tapp
A factor of production (input) that cannot be changed in the short-run.
A mathematical relationship between inputs and output. In the context of economics of the firm, cost/profits can be determined through cost/profit functions.
A factor of production (input) that depends upon the level of production. Variable inputs change depending upon how much we choose to produce.