索引顺序访问方法（ISAM, Indexed Sequential Access Method）最初是IBM公司发展起来的一个文档系统，可以连续地（按照他们进入的顺序）或者任意地（根据索引）记录任何访问。每个索引定义了一次不同排列的记录。现在这个概念用在许多场合：

在ISAM系统，数据组织成有固定长度的记录，按顺序存储的。

ISAM stands for Indexed Sequential Access Method, a method for indexing data for fast retrieval. ISAM was originally developed by IBM for mainframe computers. Today the term is used for several related concepts:

In an ISAM system, data is organized into records which are composed of fixed length fields. Records are stored sequentially, originally to speed access on a tape system. A secondary set of hash tables known as indexes contain "pointers" into the tables, allowing individual records to be retrieved without having to search the entire data set. This is a departure from the contemporaneous navigational databases, in which the pointers to other data were stored inside the records themselves. The key improvement in ISAM is that the indexes are small and can be searched quickly, thereby allowing the database to access only the records it needs. Additionally modifications to the data do not require changes to other data, only the table and indexes in question.