According to Urban Dictionary, the definition of a third wheel is:

One who deters the socialization of a couple, perhaps when being invited out of pitty [sic] or through a feeling of duty.

Related words to third wheel from Urban Dictionary include:

Awkward, tag-along, annoying, loser, loner, etc.

Google Images describes a third wheel with some of the following pictures:

How many of those third wheelers look happy? That's right, none of them. (Granted, Harry Potter had every right not to be happy since he just witnessed Buckbeak's "death"). On top of that, I don't know anyone that enjoys being referred to as "annoying" or a "loser."

Since I am an honorary third wheel, I find all of the previous descriptions to be false representations of the "single best friend." Anyway, who decided that being a third wheel has to be a bad thing?

When my best friend and her current boyfriend started dating a little over a year and a half ago, I did worry that I would be left peddling along on my unicycle of singleness as my best friend hopped aboard the tandem bike of love. That however, has not once been the case. Upholding the best friend duty of "sisters before misters," or "chicks before—," (well...you know) my best friend, and her boyfriend for that matter, have made sure that when I am third wheeling, I never feel like a Harry, but instead, more like a Cameron.

It is incredibly important to become friends with your best friend's boyfriend or girlfriend in order to achieve the "Cameron-level" of third wheeling. This optimal level brings happiness to everyone involved, for there is no awkwardness or need to choose who gets to spend time with who. This is also beneficial to your best friend since you are in the prime position to make sure all surprises and special plans are perfectly set to make your best friend happy, and who doesn't want their best friend happy? So really, when it comes down to it, third wheeling isn't to benefit you, it's to benefit the couple.

Of course, it is important to know your boundaries; the third wheel can't always be present. The more aware you are of your limits, the less likely it is that the couple will describe you as a "tag-along" or "loner." However, third wheeling is a group effort—it requires patience, a great best friend and for your best friend's partner to be understanding and accepting. Put all these things together and you can avoid the unicycle of solitude while being able to spend time with your best friend, get to see a couple extra movies and eat at a few good restaurants. So when you're third wheeling, be a Cameron.