Before revision:  A context-free language L is letter-monotonic when L \subseteq a_1^*a_2^* \cdots a_\ell^* for some finite \ell > 0 where each letter is distinct.
After revision:  A context-free language \mathcal{L \subseteq a_1^*a_2^* \cdots a_\ell^* for some finite \ell > 0 where each letter is distinct.