I would say that tenkara is perhaps a bit more broadly and loosely defined in America than has been conceived of in Japan. It is for Americans fixed line fly fishing. Japan has a greater focus on mountain stream fly-fishing than here in America. Here a certain contingent would just as well fish for still/warm-water species as for trout.
Yet in Japan, as well as here in America, the tradition of tenkara has come to be seen as coming from the commercial fisherman of the mountainous inland. Tradition is fine to acknowledge and even revere; but fishermen more often than not are going to extend a method to a (personal) logical conclusion. Fixed line fly fishing may likely go anywhere fishing with a fly-rod & reel may go, depending only upon the limitations of the fixed-line rod and a person's desire or willingness (do you really want to fish with a 15 oz. 28 foot $900 keiryu rod for 20+ lb steelhead? Perhaps not, but I've seen it successfully done in a Shimano video).
I like what one Japanese fisherman wrote about tenkara. Tenkara rods did not make tenkara. Tenkara made tenkara rods. Lighter keiryu rods, and to a lesser extent seiryu rods, are used as well by modern-day Japanese fishermen in the tenkara method. So what is tenkara in America? Mostly likely whatever any one person chooses to make of the sport for themself.