In the context of ethics most likely something like the capacity for suffering, or for any kind subjective experience.
That doesn't help me -- essentially, you just replaced the word "conscious" with the word "suffering" and that does not clarify much.
Let's try it this way. Here is a black box with something inside it. It does not communicate in any way that's meaningful to you. How can you decide whether it's conscious or capable of suffering? What would you need to measure or observe? What are your criteria?
This seems significant:
http://www.theguardian.com/science/2015/aug/18/first-almost-fully-formed-human-brain-grown-in-lab-researchers-claim