Ultimate Fighting Championship

The UFC (which stands for Ultimate Fighting Championship) is a MMA organization which has had some indirect dealings with certain professional wrestlers. Pro wrestlers have transitioned into UFC careers, and UFC fighters have transitioned into pro wrestling careers.

Pro wrestling to UFC
Brock Lesnar competed in the WWE from 2000 to 2004, and then in the UFC from 2008 to 2011 where he defeated Randy Couture to become its Heavyweight Champion. After he left UFC to return to WWE from 2012 onward. His tenure at UFC was sometimes referenced by announcers. He currently holds the WWE World Heavyweight Championship.

Leaving the WWE after a 2005-2014 run, the longest-reigning WWE Champion CM Punk (who defeated Lesnar in WWE prior to leaving) on 6 December 2014 signed a contract with UFC to fight a series of matches beginning in 2015, which made his wife AJ Lee (the longest-reigning Divas Champion) proud.

UFC to pro wrestling
Ken Shamrock competed in UFC from 1993 to 1996, then in WWF from 1997 to 1999, then returned to UFC from 2002 to 2006.

Tito Ortiz was in in UFC from 1997 to 2008, during which he made some sporadic appearances in the TNA promotion in 2005. Later in 2013 he had a lengthier run in the rebranded Impact Wrestling.

Rampage Jackson was in UFC from 2007 to January 2013, and then from June to August 2013 he was also involved with TNA, feuding with Ortiz to help promote their upcoming match at Bellator.