Meaning of “index” - Learner’s Dictionary

index

noun [ C ] us uk /ˈɪndeks/
LIST plural indexes

an alphabetical list of subjects or names at the end of a book, showing on what page they are found in the text:

Look up 'heart disease' in the index.
INFORMATION plural indexes
SYSTEM plural indices, indexes

(Definition of “index noun” from the Cambridge Learner’s Dictionary © Cambridge University Press)