A method is a member function in a class. The term is most commonly used with Java but actually is a general term when discussing object-oriented design. A method isn't like an ordinary procedure/function; it's part of the interface to a class. Methods are fundamental to encapsulation; the only way to modify an object should be through its methods.
In practice, in a language like C++, a method is like an ordinary function that is invisibly passed the "this" pointer (most of the time). Also, a method is allowed to access all member data inside its class and non-private data inside its base classes.
See also access specifier.
Contrast static member function