For me, it's when the seed pops out of the dirt (or whatever medium you're using), stands erect, and shows it's first set of leaves.
I only say that, because some seeds sprout in 3 days some a week and some never come up - so where do you begin? Good Question. I guess you can always state Day 1, seed started or Day 1 from sprouting.
I guess we should have a place to look up definitions?