What is hard link in unix?

A hard link in Unix is a directory entry that associates a name with an existing file on the filesystem, allowing multiple filenames to reference the same inode (the underlying data structure for a file). Unlike symbolic links, hard links point directly to the physical data on disk, meaning that changes to the file are reflected across all links. If the original file is deleted, the data remains accessible through any existing hard links, as the data is only removed when all links are deleted. However, hard links cannot span different filesystems or link to directories (with some exceptions for the root user).