Efficiency can be defined as the ratio of output to input for any given system. A better explanation could be skillfullness of avoiding waste. Google has a lot of other definitions as well.
The strictly economic aspect of efficiency is not always appropriate, although there may actually exist people who think so. See for example Efficiency in Art as an illustration of what i mean. I would argue that we need to take a broader view of what constitutes efficiency in software engineering. Clearly economic aspects are important, but suitability of functionality to the need as it changes over time should be considered more important.
An other problem with stating what is efficient in software engineering is that it is very difficult to draw the line of what is the boundary when you measure input and output. Some waste today could turn out to reduce waste later on.
If you draw your boundary too tight, and just focus on the next object or line of code you need to write, you can easily get stuck in local optima, which probably is very far off from the efficiency limit. If you make your boundary too wide, and try to embrace the world you will not either end up with a very good end result.
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