Short Definition
An index is a data structure that speeds up lookup by organizing references to data.
Intuition
An index is like the back of a book: it helps you find the relevant page without reading everything.
Technical Definition
Indexes maintain auxiliary structures such as trees, hashes, or vector search graphs to accelerate query access patterns.
Example
A database index can make filtering by email much faster than scanning every user row.
Common Misunderstandings
Indexes speed reads but can add write and storage cost.
The right index depends on query patterns.