Efficiency is about making proper tradeoffs between *multiple* resources.

Example -- when memory is at a surplus, trading memory for increased speed, knowing when to trade RAM usage for disk usage, when to walk a file iteratively versus loading it entirely into memory, and so on. Also, this includes knowing when to use proper resources for the proper problems.

Classic bad examples in the monastery involve building giant hashes to do complex searches when SQL would have been the better choice (for CPU, for speed, for RAM... in fact, all of the above...).

A lesser known form of efficiency is maintaince efficiency. That is, is your code clean and to the point, and how well can it be tweaked to accept new backends, etc. Though only tangentially related, efficiently written and organized code is easier to optimize and will tend to be more efficient when actually running.