I'm using the term as medical schools use it. For example, virtually every MD/PhD program says it expects significant research experience from its applicants. However, even the top MSTPs admit plenty of people without publications. They do not, however, usually admit people with less than one to two years of research experience. As far as MD programs go,
LizzyM works at a top 5 school and says that she estimates that less than 8% of their applicants have publications.
"Significant" just means "not trivial". A first author publication at the undergrad level is more than just significant, it's exceptional.