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Is it index or indexes?
The word index is singular. One index.
If you have more than one, you can form the plural according to Latin conventions:
indices
or according to English conventions:
indexes.
Preference varies...
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What is an index?
An index is in the back of a book that tells you what page number you will find a certain thing you are looking for.
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What is the index?
in a book there is a section that tells you where things are.
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What is in an index?
the back of a book
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Why is index cards called index?
Because they are commonly used to make indexes for topics. ie. You would use "index" cards to make an alphabetical index of all the books you've read, guys you've date, etc.