noun which may take the genitive case particle 'no'
undeveloped; underdeveloped; not yet developed
Describes something that has not yet reached full development, often used for skills, systems, regions, or organisms.
この地域は交通網が未発達だ。
This area has an underdeveloped transportation network.
彼の語学力はまだ未発達だ。
His language skills are still undeveloped.
発展途上 is used for countries or large-scale systems that are developing, implying ongoing progress. 未発達 simply states the lack of development without the nuance of active progression.
Compound of 未 (mi, 'not yet') + 発達 (hattatsu, 'development'). The reading みはったつ follows standard on'yomi.