In 2012 Ângela Nobre asked me what was effectiveness, and recorded a short clip with my answer for her Agere research project. I am quite fond of my conclusion:
Being effective means that you make time to tackle your hard-to-solve problems, by solving the easy ones very quickly.
I am Joaquim Baptista. I work as a technical writer in Portugal since 1997. Portugal does not have a tradition of technical writing, so I had to learn for myself, and I have been working in the saem company, a product company, since then.
Effectiveness is hard to define, but it is easier to define what is not effective, and solving the wrong problem is the least effective thing that you can do, because you are kidding yourself that you are doing useful work. You are not, and you are wasting resources in doing it.
If you are effective, you will identify what is exactly your problem, and then use the right tool for the right part of the problem. And if you do that, the easy parts of the problem quickly go away, and you are left with the hard parts of the problem. They are kind of the open problems that you don't quite know how to solve.
So, you might say that being effective means that you make time to handle, to tackle your open and hard-to-solve problems, by solving the easy ones very quickly.