which may be regarded as the probability amplitude for a quantum in state $b$ at spacetime point $y$ to turn into a quantum in state $a$ at spacetime point $x$, in the given state that the fields are in, which is defined thereby (see at state in AQFT).

Therefore often such supergravity solutions equipped with some extra data that makes them consistent CFT backgrounds at higher order are referred to as vacua for string theory. But this is in general a coarse approximation. The full vacua are the full 2d SCFTs that define the worldsheet theory of the string.

The collection of all string vacua, possibly subject to some assumptions, has come to be called the landscape of string theory vacua.