index

An index is a set of pointers that provides faster access to data in rows of a table than a table scan. The concept of a table index is similar to an index at the back of a book; index entries make it much faster to find data than starting at the beginning of the book and scanning until you find the information you are looking for. Indexes can also enforce uniqueness on rows in the table. There are two types of indexes: clustered and nonclustered.