modularity

[moj-uh-lar-i-tee, mod-yuh-]

noun

the use of individually distinct functional units, as in assembling an electronic or mechanical system.

Origin of modularity

First recorded in 1935–40; modular + -ity

Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Debugging of that unit was exceedingly laborious because of the lack of modularity in its components.
On-Line Data-Acquisition Systems in Nuclear Physics, 1969
H. W. Fulbright et al.

1909, from modular + -ity.