An Inode is an integer-like type (but not necessarily an integer--it has constant size and is sortable, but might be a tuple of ints or a long long) which represents an inode (lowercase I) in the virtual POSIX filesystem. All virtual filenames represent a path through directories to an Inode, just like POSIX.
Inodes are often used as keys in key-value pairs in databases. In this document, the term `Inode' refers either to an Inode itself, or to some value of another type in a database indexed with an Inode key. Which meaning is actually used should be clear from context.