Safety can be formally defined in various ways. See https://cs.stackexchange.com/questions/93798/what-is-a-safe-programming-language https://cs.stackexchange.c

Safety can be formally defined in various ways. See https://cs.stackexchange.com/questions/93798/what-is-a-safe-programming-language https://cs.stackexchange.com/questions/93798/what-is-a-safe-... for a decent overview. The article is likely referring to memory safety.