Definition:Homogeneous (Model Theory)
Let $T$ be an $\LL$-theory.
A model $\MM$ of $T$ is $\kappa$-homogeneous if for every subset $A$ and element $b$ in the universe of $\MM$ with the cardinality of $A$ strictly less than $\kappa$, if $f: A \to \MM$ is partial elementary, then $f$ extends to an elementary map $f^*: A \cup \set b \to \MM$.
We say $\MM$ is homogeneous if it is $\kappa$-homogeneous where $\kappa$ is the cardinality of the universe of $\MM$.
$\MM$ is homogeneous if and only if it has some infinite cardinality $\kappa$ and for every $A \subseteq \MM$ with $\card A < \kappa$, each partial elementary map $f: A \to \MM$ extends to an automorphism.
The equivalence of this definition is proved in Homogeneous iff Partial Elementary Maps Extend to Automorphisms.