Monolingual learner's dictionary
A monolingual learner's dictionary (MLD) is designed to meet the reference needs of people learning a foreign language. MLDs are based on the premise that language-learners should progress from a bilingual dictionary to a monolingual one as they become more proficient in their target language, but that general-purpose dictionaries (aimed at native speakers) are inappropriate for their needs.
Source: Wikipedia — Monolingual learner's dictionary (CC BY-SA 4.0)