I hate when I come across symbols I can't recognise or describe because it's nigh-on impossible to google for them, so I hope it's okay to post this here. Does anyone know what this symbol means within the realm of Set Theory/Predicate Logic:
In this context "WidgetNodule" is a state with the following specification:
This is in the realm of formal specification in computer science: the notation you are looking at is the Z specification language. Z is used for specifying systems by describing their states and state transitions in (a typed) set theory. In Z, $\Xi S$ is an abbreviation meaning that the part $S$ of the state is unchanged by a transition: in this case $\Xi\mathit{WidgetNodule}$ encapsulates the equation $\mathit{seen}' = \mathit{seen}$.