Haecceitism

As I've argued, the conjunction of actualism, an attractive view about the nature of possibility, and the plausible thesis that some possible objects are disjoint entails haecceitism. Moreover, if, as the foregoing considerations suggest, there are no serious objections to haecceitism, then the charge that either actualism or the thesis in question is false because their conjunction entails haecceitism is unjustified. Thus, we have a plausible argument for haecceitism.

Source: wiktionary

[D]espite the commonplace (if sometimes implicit) presumption of haecceitism underlying the work of [Gottfried Wilhelm] Leibniz and his commentators, there is remarkably strong pressure in Leibniz's mature philosophy toward anti-haecceitism.

Source: wiktionary

The haecceity of an individual x is the property of being identical with x, i.e. the property of being that very individual. [David] Kaplan defines Haecceitism as the doctrine that we can meaningfully ask whether a possible individual that exists in one possible world also exists in another without taking into account the attributes and behavior of the individuals that exist in the one world and making a comparison with the attributes and behavior of the individuals that live in the other world ... [the] doctrine that holds that it does make sense to ask – without reference to common attributes and behavior – whether this is the same individual in another possible world, that individuals can be extended in logical space (i.e., through possible worlds) in much the way we commonly regard them as being extended in physical space and time, and that a common "thisness" may underlie extreme dissimilarity or distinct thisnesses may underlie great resemblance, ...

Source: wiktionary

Haecceitists maintain that individuals have nonqualitative essences, or haecceities, and that qualitatively indiscernible worlds can be indistinct; but they make an even stronger claim. We can take haecceitism to be the view that things have nonqualitative essences, but do not have any qualitative properties essentially. […] According to the kind of haecceitism required by [Robert] Stalnaker, in order for some individual at another possible world to be you, it needs only your haecceity and it could have any other properties. For instance (to borrow an example from Lewis), you could have been a poached egg.

Source: wiktionary