Non-elementary classes of representable posets
  A poset is $(\omega,C)$-representable if it can be embedded into a field of sets in such a way that all existing joins, and all existing \emph{finite} meets are preserved. We show that the class of $(\omega,C)$-representable posets cannot be axiomatized in first order logic using the standard language of posets. We generalize this result to $(\alpha,\beta)$-representable posets for certain values of $\alpha$ and $\beta$. 
Rob Egrot
math.LO