Why is a static method considered a method?

In Java, a user-defined class is actually an instance of a subclass of java.lang.Class.

In this sense, static methods are attached to an instance of a conceptual class: they are attached to an instance of a subclass of java.lang.Class.

With this in mind, the term "class method" (an alternate name for Java's static methods) begins to make sense. And the term "class method" can be found in many places: Objective C, Smalltalk, and the JLS -- to name just a few.

Static methods are not exactly functions, the difference is subtle, but important.

A static method using only given input parameters is essentially a function.

But static methods may access static variables and other static functions (also using static variables) so static methods may have a state which is fundamentally different to a function which are by definition stateless. (ADDENDUM: While programmers are often not so strict with using "function" as definition, a strict function in computer science can access only input parameters). So defining this case of accessing static fields it is not valid to say that static methods are always functions.

Another difference which justifies the usage of "static method" is that you can define in C derivates global functions and global variables which can be accessed everywhere. If you cannot access the class which contain static methods, the methods are inaccessible, too. So "static methods" are limited in their scope by design in contrast to global functions.

The simple answer is that when Java decided to call everything a "method", they didn't care about the distinction between a function and a method in theoretical computer science.

This quote from 8.4.3.2 may help:

A method that is declared static is called a class method.

A method that is not declared static is called an instance method [...].

• Class methods: associated with a class.
• Instance methods: associated with an instance.

Java just wants you to "think object-oriented". Also, static methods have access to a surrounding scope which may include state. In a way, the class is like an object itself.