 Definable element of a first-order structure

ID: definable-element-of-a-first-order-structure

An element

a

of a structure

M

is definable without parameters when some first-order formula

φ (x)

uniquely identifies it:

M ⊨ \forall x (x = a \leftrightarrow φ (x)) .

 New to topics? Read the docs here!