The OpenQASM standard allows the definition of so-called opaque gates, which are simply placeholders for future definitions of a gate, meant to be used in situations where the gate's operation is currently unspecified. If you know C or C++, you could think of an opaque gate as a header declaration. The syntax is the same as for a user-defined gate, but without the body. Opaque gates are not used within the IBM QX, and are beyond the scope of this book.