Definition
(Absolute formula)
.
We say
φ
is absolute for
M
if for all
x
1
,
…
,
x
n
∈
M
, we have
M
⊨
φ
(
x
1
,
…
,
x
n
)
⟺
φ
(
x
1
,
…
,
x
n
)
is true
.