Manual:$wgSecretKey

Details
This should always be customised in LocalSettings.php

This value is used to generate 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. However, this wasn't fully deprecated until 1.14alpha (r40323).