Representable posets
  A poset is representable if it can be embedded in a field of sets in such a way that existing finite meets and joins become intersections and unions respectively (we say finite meets and joins are preserved). More generally, for cardinals $\alpha$ and $\beta$ a poset is said to be $(\alpha,\beta)$-representable if an embedding into a field of sets exists that preserves meets of sets smaller than $\alpha$ and joins of sets smaller than $\beta$. We show using an ultraproduct/ultraroot argument that when $2\leq\alpha,\beta\leq \omega$ the class of $(\alpha,\beta)$-representable posets is elementary, but does not have a finite axiomatization in the case where either $\alpha$ or $\beta=\omega$. We also show that the classes of posets with representations preserving either countable or all meets and joins are pseudoelementary. 
Rob Egrot
math.LO