0- Functions are first class (objects). That is, everything you can do with "data" can be done with functions themselves (such as passing a function to another function).
- Recursion is used as a primary control structure. In some languages, no other "loop" construct exists.
- There is a focus on LISt Processing (for example, the name Lisp). Lists are often used with recursion on sub-lists as a substitute for loops.
- "Pure" functional languages eschew side-effects. This excludes the almost ubiquitous pattern in imperative languages of assigning first one, then another value to the same variable to track the program state.
- FP either discourages or outright disallows statements, and instead works with the evaluation of expressions (in other words, functions plus arguments). In the pure case, one program is one expression (plus supporting definitions).
- FP worries about what is to be computed rather than how it is to be computed.
- Much FP utilizes higher order functions (in other words, functions that operate on functions that operate on functions).
