Which term describes an adversary's or penetration tester's ability to establish access to the target environment at-will and undetected?

The term that accurately describes an adversary's or penetration tester's ability to establish access to the target environment at-will and undetected is persistence. Persistence refers to the techniques and strategies employed by an attacker to maintain their foothold in a system after initial access has been gained. This can involve creating backdoors, leveraging legitimate credentials, or employing various methods to ensure that access can be re-established, even if the initial vectors of attack are discovered and mitigated.

The essence of persistence is the ability to return to the target environment continuously, thus allowing the adversary or tester to carry out further actions without being detected. This concept is critical in both offensive and defensive security practices, as it emphasizes the importance of ongoing vigilance and monitoring within a cybersecurity framework. By understanding persistence, organizations can better defend against such ongoing threats and enhance their security posture.