Fixed Line Fishing is the earliest form of fishing with rod and line. It is fishing with the line tied to the rod tip. No reel is used.
At least in the US, the term now generally refers to fishing styles derived from Japanese tenkara and keiryu fishing techniques. There is much more to it than that, but first a word about tenkara and the rise of the term "fixed line fishing."
Tenkara is a traditional Japanese fly fishing style. Some Western sources describe it as "the" traditional Japanese fly fishing style, but tenkara was not the only one. Fishing with flies for chubs and a few other species in lowland streams has been done for hundreds of years in Japan, and was documented much earlier than tenkara, but it is not called tenkara. The word tenkara, as it is used in Japan, refers only to fishing for trout and char in mountain streams. That narrow definition of tenkara is what gave birth to the term fixed line fishing in the US.
Although there were a handful of people in the US fishing with tenkara rods before 2009 (myself included) the general awareness of tenkara in the US stems from April 2009, when Daniel Galhardo formed Tenkara USA. Daniel imported tenkara rods and, with equal parts of vision and determination, brought a completely new style of fly fishing to the US.
Early on, many people mocked tenkara, saying it wasn't new and was no more than cane pole fishing. To be sure, cane pole fishing is also a form of fixed line fishing. After all, the line is tied to the rod tip, but in almost all cases, people who fish with cane poles also fish with bobbers and bait - which definitely does not fall into the category of tenkara. For that matter, even the cane pole fishing with flies for brookies in the Smokies, which had been done for generations, is sufficiently different than tenkara as it is practiced in Japan that no one should call it tenkara.
Galhardo tried to keep tenkara in the US true to tenkara in Japan, but that effort ultimately failed. The idea that it was only for trout and only for mountain streams lasted a few weeks - until someone posted photos of truly braggin' size bluegills he'd caught with his new tenkara rod. The genie was out of the bottle.
The geography of the US is too different from Japan. The fishing opportunities are too different. The culture is too different. People here quickly realized that fishing with a tenkara rod was a lot of fun, whether for trout in a mountain stream or bluegills in a farm pond. Fishing with a tenkara rod became known as tenkara, no matter where it was done and no matter what fish were caught.
I started importing Japanese keiryu rods (fixed line rods used for bait fishing in the same Japanese mountain streams) in 2012. Most Americans use keiryu rods to fish with flies, using the same tenkara lines and in many cases the same flies and techniques that tenkara anglers use.
Keiryu rods are designed for use with split shot and they tend to be stiffer than tenkara rods, particularly in the midsection. That makes them a better choice for people who wish to fish weighted nymphs, which many Americans do. The use of keiryu rods for fishing with flies also came to be called tenkara.
"Fixed line fishing" or "fixed line fly fishing" is the the term that many purists maintain should be used for any fishing done with tenkara rods that is at all different than the the tenkara fishing done in Japan (only for trout, only in mountain streams and only with a single unweighted wet fly).
The fact that some tenkara anglers in Japan use weighted flies, and some anglers in Japan use tenkara rods in lowland streams to catch fish other than trout did not change their view - the purists said what those Japanese anglers were doing wasn't tenkara either (even though that's what the Japanese who were doing it called it).
Perhaps the purists are right. After all, tenkara as practiced in Japan really is pretty limited. There are lots and lots of fish that take flies but aren't trout. There are lots and lots of places to fish that aren't in the mountains. There is a much, much longer history of fixed line fishing outside of Japan - that was never called tenkara.
The oldest written account of tenkara in Japan dates from the 1800s. The oldest written account of fishing with a fly in Japan dates from the 1600s. The earliest written account fly fishing anywhere described fixed line fly fishing around 200 BC - a full 1400 years earlier!
Fixed line bait fishing is older yet. Accounts from Greece and Egypt suggest it has been done for thousands of years. Recently, people in the US have been adopting the Japanese bait fishing rods and techniques used in mountain streams, but even more so than with fly fishing, fixed line bait fishing is much, much broader.
This page is not finished. The debate about whether what we do here in the US should be called tenkara is not finished, either. Please read the pages on tenkara and keiryu fishing and decide for yourself. (I think you might decide that what it is called isn't all that important if you have fun doing it.)