Meaning of = delete after function declaration

Deleting a function is a C++11 feature:

The common idiom of “prohibiting copying” can now be expressed directly:

class X {
    // ...
    X& operator=(const X&) = delete;  // Disallow copying
    X(const X&) = delete;
};

[…]

The “delete” mechanism can be used for any function. For example, we can eliminate an undesired conversion like this:

struct Z {
    // ...

    Z(long long);     // can initialize with an long long         
    Z(long) = delete; // but not anything less
};