  We first define the trace on a domain $\Omega$ which is definable in an
o-minimal structure. We then show that every function $u\in W^{1,p}(\Omega)$
vanishing on the boundary in the trace sense satisfies Poincar\'e inequality.
We finally show, given a definable family of domains $(\Omega_t)_{t\in
\mathbb{R}^k}$, that the constant of this inequality remains bounded, if so
does the volume of $\Omega_t$.
