IPA: /ˌkɒmplɪmɛnˈtærɪti/
KK: /kəmˌplɛməˈtɛrɪti/
The quality or state of being complementary, where two or more things complete or enhance each other.
The complementarity of the two theories helps us understand the complex nature of light.
Complementarity → It is formed from "com-" (meaning together) and "plementum" (meaning that which completes). The word "complementarity" refers to the quality of being complementary or completing each other.
Think of things that work "together" ('com-') to "complete" ('plementum') each other — that's what complementarity means.