noun which may take the genitive case particle 'no'
convex; bulging in the middle
Describes a shape that is raised or swollen in the center, such as a convex lens or a surface that is higher in the middle than at the edges. Often used in technical or descriptive contexts.
このレンズは中高になっている。
This lens is convex.
板が湿気で中高に反った。
The board warped, bulging in the middle due to moisture.
Compound of 中 (naka, 'middle') and 高 (daka, from 高い takai, 'high'). The reading なかだか follows rendaku voicing of the second element.