Wow, just no. Sending a message to an object is more like calling your mom. Applying a function is like...applying a function. There is no real analogue for that, it's just math.
A function is well defined in mathematics; see http://en.wikipedia.org/wiki/Function_(mathematics) . Note that the definition doesn't really differ for computer science. Procedures, on the other hand, may or may not be functions according to whether they behave like them (match their properties).
Message passing can be synchronous, and function calls asynchronous
Also consider remote procedure calls.