The quality of being impossible or difficult to detect.
2006, David Salomon, Coding for Data and Computer Communications, →ISBN, page 345:
In particular, embedding capacity, robustness, and undetectability are mutually conflicting and cannot all be achieved by the same algorithm.
2008, Encyclopedia of Information Science and Technology, Second ..., →ISBN:
The highest degrees of anonymity are undetectability and unobservability.
2010, Information Hiding, →ISBN, page 197:
On a high level, undetectability means that no efficient algorithm can distinguish between inter-packet delays of the legitimate traffic and steganographic traffic.