How would you answer it?
I've heard it used several different ways. One way is simply as "senior" and "junior", which could be by no more than a day -- not a really meaningful term, except when you're figuring out who sits to the left of who when you line up. Another way is as senior enough where they would have relevant clues to impart to you, or junior enough where you might have something to say that would help them. This is more useful, but I prefer a third usage, where "sempai" isn't just someone who is senior enough to mentor you, but who actively does so -- your sempai is/are those senior students who take the trouble to teach you, guide you and look out for you.
I've got two people who answer this description in my current dojo. Neither is the highest ranking student there (one's shodan, one's nidan), but among the yudansha, they're the ones who have extended themselves to help me, push me, lead me on to more challenging things and hold me to a higher standard. One has a particular affinity for weapons, and since I have prior weapons training in a different style, he usually works with me when we partner off and takes me a bit further through the kata than I would probably get otherwise. The other spent hours, literally hours, after class every week through last summer, helping me and others prepare for our upcoming kyu test. They encourage, they push, they know what I can and can't do (and what I should be doing next). In short...they're a lot more than the people who sit to the left of me in line.