What exactly defines a safety school?

I agree that many publics can be good choices for safeties, including SJSU and others in the Cal State system. However, one still needs to look at the chance of a particular student being accepted, which may be completely different from an estimation based on the school's overall stats and overall acceptance rate.

For example, SJSU had a 64% acceptance rate in the most recent CDS and admits almost entirely on stats + transcript. In the most recent CDS, the 25th to 75th percentile SAT ranges were 1030-1260 and average GPA was 3.52. Nevertheless, a particular applicant with a 4.0 + 1600 SAT may be rejected, and a particular applicant with a 2.75 + 800 SAT may be accepted. Consider the following 2 SJSU applicants for 2020.

Applicant 1 – SF resident with 4.0 GPA (>4.0 is possible with eligible honors) and 1600 SAT – Only applies to CS. The applicant sees that 4.0 GPA + 1600 was above the EI stat index for 2019, so he considers SJSU a safety. The EI stat requirement for freshman CS increased to 4825 for 2020, so he does not meet the new increased EI and is rejected from his safety.

Applicant 2 – SF resident with 2.75 GPA (no grades below C-) and 800 SAT. Applies to Applied Math: Computational. Meets EI stat requirement for Applied Math and is accepted.