noun, noun which may take the genitive case particle 'no'
under development; in progress
Used for projects, plans, systems, or areas that are currently being developed or are in the process of being built/improved. Often appears in business, technology, and urban planning contexts.
このアプリはまだ発展中です。
This app is still under development.
発展中の地域には新しい道路が建設されている。
New roads are being built in the developing area.
進行中 is a broader term for 'in progress' and can apply to any ongoing activity, whereas 発展中 specifically implies development or evolution.
Compound of 発展 (development) + 中 (in the middle of). The reading はってんちゅう follows standard on'yomi patterns.