The Hidden modifier may be used to hide a property or a method from casual discovery. Members marked as Hidden are still visible when using Get-Member, and may still be invoked. In many respects, this is similar to the DontShow property of the Parameter attribute: it hides the member from IntelliSense and tab completion, but does not prevent use. 

In the following example, the Initialize method is hidden:

class MyClass {
    [String]$Property

    MyClass() { $this.Initialize() }

    Hidden [Void] Initialize() {
        $this.Property = 'defaultValue'
    }
}

By default, the Initialize method will be hidden from view. Using Get-Member with the Force parameter will show the method:

PS> [MyClass]::new() | Get-Member Initialize -Force

    TypeName: MyClass

Name          MemberType    Definition
----          ----------    ----------
Initialize    Method        void Initialize()

It is not possible to make members private in PowerShell at this time.