List of 'known-good' programs, email addresses, servers or websites
Whitelisting refers to the use of a list of known-good items (such as files, mailservers, websites etc.) to which to allow access, while all others are blocked. This is the opposite to blacklisting, in which access is allowed to all but the known-bad items.
Whitelisting is implemented in some web-filtering systems (sometimes referred to as 'Walled Gardens'), allowing access to only a carefully screened selection of websites, ensuring maximum security from malicious sites while allowing access to information required for productivity.
When applied to a local system a file access filter is implemented in much the same way as in traditional anti-malware software. The filter checks any executable code requesting permission to run against a list of approved software, blocking anything that does not match up. Generally some sort of integrity checking, such as checksumming files, is used to ensure trusted items are not tampered with.
The 'whitelist' itself is the list of trusted items, which is often dynamic to allow regular and rapid updating.