One way to hold this word: two things laid side by side, and they match. Two reports say the same thing. Two twins carry the same trait. A patient and a doctor land on the same plan. And a word-list is the same picture, just done thousands of times over: every place one word appears is pulled out and lined up so the uses sit together and can be compared.
From the word’s history
The packet's etymology tree traces the word through Old French concordance and Late Latin concordantia to Latin concors, built from con- (together) and cor (heart) — 'of one heart'. That root sits right under the modern core: things that agree, and lists that bring matching uses together.
封包裡的字源樹把這個字往回追到古法文 concordance、晚期拉丁文 concordantia，再到拉丁文 concors——由 con-（一起）加 cor（心）組成，字面是「同一顆心」。這條線索正好對上它今天的核心：兩邊對得起來；而一本索引做的，就是把同一個字的各處用法對齊在一起。
Wiktionary, "concordance"
登入就可以看每一支怎麼連回核心畫面。