Short version: The source of precious fangirl tears.
The long version... is pretty darn long! Fanlore defines an OTP as an abbreviation of One True Pairing. That means that the shipper has one 'ship' that they're really invested in, though the term has changed a little over the years (if you're not familiar with shipping, check out this card). There are a lot of different things this can mean though, which I'll explore here.
I've really enjoyed making cards to explain the sometimes confusing terms for people new to fandom. You can see the rest of them here! You might already have an OTP and just not realize it!
Are you ready to find out?
One True Pairing
The Marvel fandom has a lot of these. An OTP can mean that there's one ship above all others that the shipper cares about. However, the term has come to mean something more all-encompassing as fans become more inclusive. Instead of having one OTP, many shippers have several.
Two major OTPs in the Marvel fandom right now are Stony and Stucky (pictured above). The first pairing (Steve Rogers and Tony Stark) was very popular before the MCU films because of the dynamic the characters had in the comics. After the release of Winter Soldier, fandom shifted and began shipping Steve Rogers with Bucky Barnes. These two ships are not essentially mutually exclusive. Though often not within the same fic or fanart, fans can and do ship both as an OTP.
Clintasha (Clint Barton and Natasha Romanoff) were extremely popular after the release of the first Avengers movie. The costume piece Scarlett Johannsen added to the character- the arrow necklace- was often cited as textual evidence of some sort of relationship beyond the Avengers. Fans also ship Clint x Coulson and Natasha x Bucky.
Science Bros (Tony Stark and Bruce Banner) also blossomed after the release of the first Avengers film. Fans adored the banter and rapport the two characters built up during the movie. Another OTP is Pepper x Tony, particularly post Iron Man 3.
Thorki (Thor and Loki) became popular after the first Thor movie. Fans responded to the strong emotional drama of the (adopted) brothers' struggles. The night and day dynamic they have has been fodder for OTPs for a long time; these shippers were carrying on the tradition. Though less popular, another OTP is Jane x Thor.
Spideypool (Wade Wilson and Peter Parker) is popular because of the dynamic the characters have in the comics. Deadpool even refers to the popularity of the ship, which is completely in tune with the metahumor the character is known for.
Charles-x-Erik (Charles Xavier and Erik Lehnsherr) has been a popular OTP for several years, though the fandom got a boost after the release of Days of Future Past.
Are you ready for the next level?
Because the more the merrier! Similar to the term OTP, an OT3 can signify a shipper's One True Threesome, but often it is used to indicate a favorite ship, instead of an exclusive one.
The Marvel fandom has a multitude of OT3 ships including: Steve/Bucky/Peggy (pictured), Mystique/Charles/Xavier, Bruce/Tony/Pepper, Sam/Steve/Bucky, Deadpool/Spider-Man/Daredevil, Natasha/Clint/Coulson, and Bucky/Natasha/Steve.
One thing to keep in mind: Someone's OTP is *very* important to them.
And OTP is an important part of being a fan for some.
It's how many fans express their love for the original media, either by reading and writing fanfiction, creating fanart and other fanworks, composing meta and theories about the pairing, or just chatting with other fans who are also invested in the ship. You're not required to like the ship, or agree with it, but it's considered extremely inappropriate to go into spaces dedicated to an OTP just to bash it. These spaces include tags, the comments on fic, and chatrooms dedicated to an OTP (this is not an exhaustive list).
Not feeling it? You're in luck!
The fandom community has provided a term for fans to use when they encounter a ship they don't like: The NOTP. You can probably guess what it means. The NOTP is a pairing that you. Just. Don't. Like. This can be for any reason. Maybe you don't see the dynamic working like other fans do. Maybe you just don't like one of the characters involved. Maybe something about the pairing makes you uncomfortable because of the power exchange involved.
All your reasons are valid! Just remember:
So don't go hating on someone else's OTP.
It doesn't make for a healthy community!
Fandom can be a strange place if you're not familiar with the territory. Hopefully with this new understanding of the terminology you can safely traverse the world of fandom, and maybe pick up a few ships of your own (please do, we will have SO MUCH to talk about!).