What is the efficiency of a programming language?

Programmers often talk about a language being efficient or otherwise. C++, in particular, has been developed with the explicit goal of efficiency and, at the same time, has a reputation in some circles of being inefficient. How can this be?

Efficiency can mean different things in different contexts or to different people. For example:

• C++ design follows the principle of zero overhead: with a handful of exceptions, you don’t pay any runtime cost for any feature you do not use just because it is present in the language. In this sense, it is as efficient as a language can be.
• You obviously have to pay something for the language features you do use, at least if they translate into some runtime work. C++ is very good about not requiring any runtime code for doing work that can be done during compilation (although the implementations of the compilers and the standard libraries vary in their efficiency). An efficient...