What exactly defines a safety school?

I would have to agree with this.

Many large flagships with acceptance rates in the 40%-50% range are often safeties for students with GPAs above 3.8, and tests which are in the top 1%-5%. For example, UMN, which has an acceptance rate of around 45%, accepted every student from my kid’s high school in the range of GPA > 3.79, SAT > 1500. That entire upper right hand corner of the Naviance scattergram was solid green checks, with nary a red X to be seen.