What is the default package in which my classes are put if I don't specify it?

Let's assume I have a file named Main.java with the following code:

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello world");
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}

Is it put in a specific package, in (maybe?) an unnamed package?

A class that is not in a named package is in an unnamed package. Thus the full class name is Main.

Such classes cannot be used from a named package, except via reflection.

The JLS says that:

Unnamed packages are provided by the Java SE platform principally for convenience when developing small or temporary applications or when just beginning development.

Java does not have namespaces, it has packages. And yes, classes with no package declarations are implicitly part of an "unnamed package", often also called "default package". However, since it's not possible to import classes from an unnamed package and since the language spec explicitly allows implementations to have different rules about whether and how classes in unnamed packages are visible to each other, it's generally a good idea to put all classes in named packages except for experimental code.

According to the JLS it's called:

7.4.2 Unnamed Packages

A compilation unit that has no package declaration is part of an unnamed package.