A motive over a field $k$ is of abelian type if it belongs to the thick and rigid subcategory of Chow motives spanned by the motives of abelian varieties over $k$. I understand that this is the smallest thick strictly full rigid tensor subcategory of $\mathcal{M}_{rat}$ that contains Artin motives and motives of abelian varieties.
What is the definition of thickness in this context? Of course I know that thickness is stability under extensions, but how does one checks this in this context? $\mathcal{M}_{rat}$ is just pseudo-abelian, and not abelian nor triangulated, so I am a bit confused.
