'Martial Art' is an American term as can be seen by the English letters used to display it on your computer screen. It probably popped up sometime in the states in the early 20th century and referred to whatever Japanese or Okinawan systems had first crossed over the pond.
Today it's a term we use to lump together the practice of Karate, Judo, Kempo, system-of-the-month, what-have-you. When you look at the different martial systems of the world, it would be best to look at them from their own languages to better understand where they come from.
Read up on a little of Dave Lowry's works if you are interested in the closest Japanese tranlation of 'martial arts'...being Bu Jutsu. You'll find that looking at the meaning of Bu Jutsu, Mr. Lowry has separated Karate from being a Martial Art to a system of self defense.
But this has little to do with Karate's effectiveness and more to do with consistent and accurate terminology. To the original post, I think in America, Martial Art has come to mean a system of armed or unarmed fighting designed for self defense and we use it to describe almost all systems here today. (Look at MMA).
There's always going to be shades of gray and people who will say, well, so-and-so-art really isn't a martial art. I'm one of them. You just have to get the right context to see where they are coming from. For the most part, unless you are talking to a hoplologist or serious practitioner of some classical system, Martial Art will get the point of what you do across to almost any non-practitioner in the US. When speaking to each other, we should be more precise.