Absoluteness

https://en.wikipedia.org/wiki/Absoluteness

In mathematical logic, a formula is said to be absolute if it has the same truth value in each class of structures/models.

Theorems about absoluteness typically establish relationships between the absoluteness of formulas and their syntactic form.

There are two weaker forms of partial absoluteness:

• if the truth of a formula in each substructure N of a structure M follows from its truth in M, the formula is downward absolute.

• if the truth of a formula in a structure N implies its truth in each structure M extending N, the formula is upward absolute.