In a Unix-style file system, an index node, informally referred to as an inode, is a data structure used to represent a filesystem object, which can be one of various things including a file or a directory. Each inode stores the attributes and disk block location(s) of the filesystem object's data. Filesystem object attributes may include manipulation metadata (e.g. change, access, modify time), as well as owner and permission data (e.g. group-id, user-id, permissions).
A Linux directory lists other filesystem objects by name, normally identifying the listed object by referring to its inode. The directory contains an entry for itself, its parent, and each of its children.
Etymology
The reason for designating these as "i" nodes is unknown. When asked, Unix pioneer Dennis Ritchie replied:
In truth, I don't know either. It was just a term that we started to use. "Index" is my best guess, because of the slightly unusual file system structure that stored the access information of files as a flat array on the disk, with all the hierarchical directory information living aside from this. Thus the i-number is an index in this array, the i-node is the selected element of the array. (The "i-" notation was used in the 1st edition manual; its hyphen was gradually dropped.)
Also Maurice J. Bach writes:
The term inode is a contraction of the term index node and is commonly used in literature on the UNIX system.
—Maurice J. Bach, The Design of the Unix Operating System, 1986
