Ask someone this question and you'll probably get a variety of answers. For such a popular format why is there no definitive answer? Try taking a look at the definition that Wikipedia provides of smooth jazz. What a bonafide mess!!! In most cases, smooth jazz is defined as a modern form of jazz developed in the 1970s that has elements of rhythm and blues (R&B). I think the most accurate definition I've come across has been found here on Squidoo. Yes, it is true that in many instances it's a melodic line with an R&B backbeat, but there's a little more to it. In fact, there's a lot more to it. Smooth jazz recordings feature a controlled studio recording environment, but still retain the elements of improvisation.
While many jazz purists criticize smooth jazz, this genre of music is a great entry point for facilitating into jazz in general. Smooth jazz has many elements of pop, rock, latin, funk, soul and much more. Due to the many different genres of music that are incorporated into smooth jazz, individuals that would not listen to jazz find themselves becoming a fan of smooth jazz.
I welcome your comments on this broad question.