The term "Americana" gets used more pervasively on this site than does "billion" by our congressmen, and I tend to wonder if either word is truly understood. "Americana" appears to include a billion different things and exclude very few. The more I read and hear the word "Americana" as applied to music, the more I understand where the "whatever that is" used by the No Depression magazine came from, though the answer is no more clear now than it was then.
Before continuing, let me say that there are obviously more consequential topics of interest in music that will foster interesting discussion, but this just happens to be a pet peeve, a pebble in my shoe if you will.
My notion is that "Americana" is a fairly recent wide net marketing phrase thrown retroactively over a very diverse group of roots music going back decades, and that more than anything else, it is a playlist and means of promoting roots music. On the other hand, others have called it a genre, which seems out of line with the definition of genre, which is: "a particular kind or style of art or literature" according to the Oxford American Dictionary. Leaving out arguments about the ancient origins of all music, I'd argue that the primary colors of American music are jazz, blues, folk and country. These are genres. But what do you call the blends? That's the tricky part.
Is Americana a genre? A sub-genre? Neither? Is it a marketing device? A playlist? The latest way to describe country rock? Folk rock? Alt Country? No Depression? Does it mean all music with a fiddle? Only music with a fiddle? All music liked by those who play and like fiddle music? Is it anything old timey and rustic sounding? Does it include punk? Does it exclude the blues and jazz? Is bluegrass bluegrass and/or Americana? Is it only Gram, Lucinda, and Buddy (okay, that last one is tongue in cheek, they are as ubiquitous on this site as is "Americana", if not moreso, but better understood). Circle all that apply, if you can.
In recent years we've had new terms describing roots music crop up: Alt Country, No Depression, Americana. Are they distinctions without difference, do they have separate meanings, or are they simply the latest phrases that have caught on to describe roots music? Where'd the phrase cowpunk go? Morphed into one of the above?
According to Wikipedia, the definition of "Americana music" is as follows:
Americana is an amalgam of roots musics formed by the confluence of the shared and varied traditions that make up the American musical ethos; specifically those sounds that are merged from folk, country, rhythm and blues, rock and roll and other external influential styles. Americana is popularly referred to, especially in print, as alternative country, alt-country or sometimes alt.country. 
Americana as a radio format
Americana, as defined by the Americana Music Association (AMA), is "American roots music based on the traditions of country. While the musical model can be traced back to the Elvis Presley marriage of 'hillbilly music' and R&B that birthed rock 'n roll, Americana as a radio format developed during the 1990s as a reaction to the highly polished sound that defined the mainstream music of that decade." Because of listener interest in the artists who do not fit as comfortably in the country or rock genres, a radio format called "Americana" was developed by the AMA and reported by R&R (Radio & Records, a radio trade publication). Born out of the Triple A, non-commercial, country and other formats, the Americana format is the sum of the parts that have showcased Americana music since its inception.
The AMA grew out of the format as an effort to bring all Americana music supporters, performers, and professionals together to expand the visibility and viability of the music. The radio format, including the term "Americana," began in early 1995 through the efforts of Rob Bleetstein of San Francisco, and Jon Grimson of Nashville. Bleetstein became the first Americana chart editor as Gavin (a former radio trade publication) magazine created the first Americana radio chart which was published on January 20, 1995. This came about when KFAT (defunct) radio in Gilroy, California went off the air, and Bleetstein went to the Gavin Report, asserting that they were missing a category of music. He described the KFAT format, which had the widest playlist of any station in the country, and most of whose artists whose music would come to be known as Americana. Bleetstein worked closely with KFAN "Texas Rebel Radio" in Texas and KPIG in California in developing the Americana format. Both stations had been on the air with their own versions of an independent format for several years and had been instrumental in the development of the AAA format as well. The publisher agreed and gave Bleetstein the job of creating and running the chart. Grimson coined the term Americana and became the first Americana radio promoter after having promoted the music previously at Warner Brothers Records Nashville, and promoting those releases that WB worked to radio formats outside the mainstream country stations. The AMA was later established to expand the musical format outside of strictly radio, but still including the radio aspect as well. The non-profit AMA now runs the Americana radio chart.
What about the term Alt Country? Here's what Wikipedia has on that.
Alternative country (sometimes alt-country, insurgent country, or Americana) is a loosely defined sub-genre of country music, which includes acts that differ significantly in style from mainstream or pop country music. It has been used to describe country music bands and artists that have incorporated influences ranging from roots rock, bluegrass, rockabilly, honky-tonk, alternative rock, folk rock, and particularly punk.
And Wikipedia says this about No Depression music:
No Depression in Heaven" (or simply "No Depression") is a song that was first recorded by the original Carter Family in 1936 during the Great Depression. Although A.P. Carter has frequently been credited as the author, some sources attribute the song to James David Vaughan.
Over the years the song has been recorded by artists as diverse as the New Lost City Ramblers and Sheryl Crow. Uncle Tupelo made the song the title cut of their 1990 album, No Depression. Since then, the title has become synonymous with alternative country music. What is now described as the "No Depression movement"  is covered by many fan publications including the magazine No Depression.
Obviously Wikipedia is not necessarily 100% accurate and the arbiter of all things. But a discussion has to start somewhere, and so these excerpts are offered as food for thought and starting points for anyone interested in the relatively inconsequential in the scheme of things discussion over this ubiquitous phrase.
So then, what does Americana as applied to music mean, specifically. Not vaguely.