What's my nationality?

I realize this may be a silly question, but I’ve always had trouble with this term. I know what it means, but it’s just one of those things about which I always second-guess myself. Essentially, I’m filling out some paperwork for a job for which I’ve been interviewing and there is a section that asks for my nationality. I was born and raised in Cape Verde but I’ve resided in the USA for the past ten years and I hold passports for both countries. Which one should I list as my nationality for the purposes of this application?

Thank you! :)