I-Node:    The i-node is the focus of all file activity in the UNIX file system.  There is a unique i-node allocated for each active file, each current directory, each mounted-on file, text file, and the root.  An i-node is ‘named’ by its dev/inumber pair. (iget/iget.c) Data in icommon is read in from permanent i-node on volume. 

The layout of an i-node, and the constants and macros associated with it, is defined in the include file /usr/include/sys/inode.h. 

Sun System Release 0.3