Inode
In plain terms
A data structure storing a file's metadata.
Going deeper
An inode holds a file's metadata, permissions, size, timestamps, and disk locations, but not its name. It's the file's identity record.
Under the hood
Filenames map to inodes via directory entries; running out of inodes blocks new files even with free space.
Examples
Unix/Linux filesystems, file metadata, ls -i, permissions, hard links.
Think of it like...
Like a library catalog card holding all details about a book except its title on the shelf.