Let's start a shipping war, don't care if I get hate.
Don't like my pairings, well, then you can hit the bricks.
This is my OTP, I'll go down with this ship!
I ship it! I ship it!
The One True Pairing (usually abbreviated as OTP, not to be confused with OOTP or OotP) is the couple that a fan or group of fans prefers over all other 'ships. Canon and logic do not necessarily apply, and crossover OTPs are not uncommon.
The degrees to which this is taken varies wildly. It can mean anything from "I just like Drakken/Shego best" to "I won't read Fan Fic that pairs Anya and Xander with anyone but each other" to "Cloud always and still loves Aerith and they made love to each other in the flower field at the end of the movie", to the extreme "All of the canonical evidence that Harry and Hermione are not going to get together is a lie spread by the Ron/Hermione 'shippers, who must be stopped before their evil destroys the universe". note
OTP shipping wars were second only to Subbing vs. Dubbing for heat and violence. The degree of fanaticism displayed for a particular OTP may be proportional to their distance from the Official Couple and/or the Word of God for the work in question.
In some cases, holding their preferred characters to completely different standards than the official pairing is an excellent mark of OTP shippers. If someone argues that you need to show that X and Y got married to prove they're in love, and the rings on their fingers don't count, but they know that X and Z love each other because of 'chemistry', they're definitely defending their OTP.
It is technically possible for a person to have multiple OTPs, just with no overlap between them.
Some people refer to their favourite threesome ship by the similar term One True Threesome, or OT3. Or really, "OT" appended with any number if they're even more ambitious with the number of people involved.
BroTP is used as a term for non-romantic OTPs, of the "these people are best friends and no one will tell me otherwise" variety (for instance, John Sheridan and Susan Ivanova of Babylon 5). Male/female and female/female BroTPs are sometimes also called FrOTPs.
Inversely, a NoTP is a ship that one opposes vehemently.
Often the term "OTP" is used sarcastically in parody of shipping, especially if it involves an unconventional pairing (such as the ever-popular ChadxFloor OTP), or a character and their favorite food ("Hiro/Waffles OTP"), etc etc.