Conor McGregor's defining moments in the UFC

UFC megastar Conor McGregor announced his retirement via social media on Tuesday -- likely spurning thousands of panic attacks across the MMA world in the process.

The complete fallout from this announcement has yet to be determined. Has McGregor, 27, really called it quits? During an appearance on SportsCenter, UFC president Dana White said he didn't know -- but the UFC has officially pulled McGregor from his scheduled appearance at UFC 200 on July 9 and is actively seeking a new main event.

If the Dublin native has truly decided to hang it up (and walk away from millions of dollars), he has still achieved -- by several definitions -- more than nearly anyone else has in the sport's history.

Here are the top-three defining moments from McGregor's UFC career.

3. Conor McGregor knocks out Chad Mendes in Round 2 at UFC 189, July 11, 2015

The fight was incredible and the circumstances surrounding it were unbelievable (although, that has proved to be a theme in McGregor fights). McGregor was supposed to challenge Jose Aldo, the most dominant champion in featherweight history, but Aldo withdrew less than two weeks prior to the event with a broken rib.

Rather than pull out and wait for Aldo to return, McGregor agreed to fight Mendes -- who, although coming in on short notice, represented a completely different fighter than Aldo and was easily the best wrestler McGregor had ever fought.

When told of the potential switch, McGregor, who was sleeping at a rented house in Las Vegas at the time, rolled over and said, "They are all the same," according to his coach John Kavanagh. McGregor even tried to up the stakes, offering White and UFC CEO Lorenzo Fertitta a $3 million wager that he would knock Mendes out in the second round.

UFC executives didn't take the bait, which turned out to be a wise move. McGregor stalked Mendes from start to finish and despite being taken down several times, he finished the former two-time title challenger in the second round. Just as he called it.

The event drew a live gate of $7.2 million, setting a new UFC record in the United States, breaking marks set by Anderson Silva and Georges St-Pierre.

2. McGregor knocks out Dennis Siver, leaps over cage to confront Jose Aldo, Jan. 18, 2015

This was the moment that created the McGregor-Aldo rivalry. There had been talk back and forth between them, mostly through headlines, prior to this. Aldo even attended this Boston event wearing a crown and carrying a sign that called McGregor a "joker," who was only there to make him laugh.

And that was fun, but it likely wouldn't have grabbed the attention of the sports world on its own. McGregor did that -- by knocking out Siver in the second round and sprinting immediately to the fence, then scaling it to confront Aldo in the stands. Of course, cameras were there to catch every angle of it, and the scene was transmitted across the airwaves on a UFC card that delivered exceptional ratings.

Fertitta later told ESPN.com how that one moment convinced the promotion to send McGregor and Aldo on a multimillion dollar world tour in 2015, which made stops in Rio de Janeiro, Las Vegas, Los Angeles, Boston, New York, Toronto, London and Dublin. That also happened to be McGregor's last fight on free television.

1. McGregor knocks out Aldo in 13 seconds at UFC 194, Dec. 12, 2015

The fastest knockout in UFC title fight history. Even if McGregor does return to the Octagon at some point, this could still end up forever being the fight that defines his career. Hopefully, history will remember how great Aldo was prior to this loss. Aldo had not tasted defeat in 10 years. He had eaten alive the best competitors of his era and defended his 145-pound title nine times, if you include his time in the WEC.

Of course, there are some who see a 13-second knockout as something of a fluke. Aldo has stated that numerous times since. The Brazilian has referred to McGregor's left as "a lucky punch."

McGregor had breezed his way through the division so effortlessly, however, and made good on nearly every single one of his prefight predictions. Even his critics had a hard time not being in awe of a first-round knockout against one of the best ever. And did it ever sell: UFC 194 drew a gate of $10 million, which broke the mark McGregor had set in his fight against Mendes five months prior.