Manual:$wgSecretKey

Details
This should always be customized in LocalSettings.php

This value is used as cryptographic entropy when generating user_token's to insert into the users table which is used as a persistent cookie for authentication that is resilient to spoofing.

$wgProxyKey
From 1.3 to 1.4, $wgProxyKey was the documented setting for this. In 1.4, this was marked as deprecated in favor of $wgSecretKey.