The invocation of a method with arguments.
5 + 7 is a call of the method
"_+_" with the arguments
An ambiguous method call is possible. If two modules that do not inherit from each other define distinct implementations for the same signature, and those two methods are in the inheritance path of a single third module, a simple method-name invocation in the third module or subsequent modules cannot be resolved by the compiler. To disambiguate, the compiler requires that such calls use the full method name.
References: An in-depth look at send phrases