Unlinkability of two items means that an adversary cannot determine if the items are related. It is often a goal in systems dealing with anonymity and other forms of privacy.
The unlinkability of two items is defined by Pfitzmann et al. to mean that "within the system (comprising these and possibly other items), the attacker cannot sufficiently distinguish whether these [items] are related or not." (Pfitzmann et al., Def. 4).
This could for example mean that an attacker could not determine if two texts were written by the same person or two transactions involved the same person. As such, it is mostly relevant for systems trying to ensure anonymity and related security goals.
