An index is a copy (or copies) of some or all of the searchable data in a table to a data structure that's easy to search (binary tree, etc.). As long as the in

An index is a copy (or copies) of some or all of the searchable data in a table to a data structure that's easy to search (binary tree, etc.). As long as the interviewee knows it's a copy of the data used to speed up searching, I don't see what more you're looking for (if you are looking for more). If you want to get into more detail, you'll be looking at various tree structures and search algorithms on those structures. Unless you're writing a DB, language, or something else low-level, you won't have experience with those and they are largely irrelevant for typical web development.