I use OTP to mean both pairings that I like to read/write and pairings that make the most sense to me based on my interpretation of the source material. I will read stories outside of my OTP, but if they involve one of my OTP with someone other than the other half (third, fourth, etc.) of my OTP, I still read them through the lens of my OTP. For example, I will read a Snape/Lupin story, but it will be filtered through my belief in and fondness for Sirius/Remus. If the author does not actually write Remus feeling a lack or mourning Sirius' loss, I will fill in the blanks myself. If the author writes Remus as never having loved Sirius or never having been attracted to him, or as having been in a relationship with him and feeling a lack in it that Snape fills, I consider that a weakness in characterization. Or rather, I care that's it a weakness in characterization. Authors change or ignore things in the source material all the time, but I only tend to get peeved about it in certain instances. One of those instances in when what is being ignored or changed is something of significance to my OTP.

That said, most of the time when I use the actual term OTP, I'm being very tongue-in-cheek. After all, I consider myself and my icon space the OTPiest of OTPs. *G*