It sounds like you're answering the wrong question. All previous answers have addressed you finding a word to describe the task you were doing. But this isn't what you say you're being asked for.
If they're wanting to know what new skills you learnt, they don't care whether you refactored code or wrote new features or whatever. That isn't a skill, that's a task. A skill is something like "how to program in C", or "how to code for test". Your tasks are how you learnt those skills.
Remember that you're writing this for someone else to read. Check again what you're being asked to give them. And after you've written it, pretend that you're them and read it as if it was something that had just been handed to you, using the criteria that you've been given. Then go back and rework the bits that don't fit what someone in that position would be looking for.