Any group underrepresented in [valuable activity x]relative to its distribution of talent.
You can theoretically have a distribution in which natural aptitude is equal, the distribution is skewed (more men than women) and it's not horribly inefficient, but I think if you spell out the assumptions of such a distribution, it's not especially realistic.
There is good reason to believe that natural aptitude is not equal[1][2]
[1] https://docs.google.com/file/d/0B7DMQCP3v6SZQUw1MnNWbXBUV3M/...
[2] http://www.aei-ideas.org/2012/09/2012-sat-test-results-a-hug...
What if women are equal to men in coding aptitude, but simply don't want to code? Is that still a waste that we should do something about?
Depends on why. If it really is because regardless of what social expectations there are, women are just less interested, that's harmless. But I think it's obvious that with a look at history and present attitudes, we can't assume an observed difference in interest is neutral that way.
Is that correct?