The term special guest referee refers to the ref in a match in which the usual referee is replaced with a "guest" filling in as the official.
Celebrities (such as Muhammad Ali in the main event of WrestleMania), managers and other wrestlers can "guest" as the special referee. In some cases, a special referee is put into a match which is already a different match type or stipulation (for example: Hell in a Cell with a Special Referee).
The special referee will often be biased towards or against one of the competitors or will be assigned as the Special Referee to ensure the match is called down the line.
In September 1999, in the WWF, after all the referees got sick of continuously being attacked by wrestlers, they went on strike (kayfabe), leading to other WWF workers (most notably Harvey Wippleman and Tom Prichard, along with a non-striking Jim Korderas) becoming "scab" referees until the night after Unforgiven, where Vince McMahon gave the regular referees more authority in matches (along with fining Triple H for striking one that same night).