An independent interstellar merchant ship, unaffiliated with any megacorp, polity or archailect empire.
In essence, a Free Trader is a mobile Free Zone which travels the Nexus and beyond, carrying cargo and passengers. Some are Jack-of-all-Trades, others are specialized for a single role; some crewed by many kinds of sapients, some by only a single clade or sub-clade or sub-culture, and sometimes (in the case of sentient ships) by none.
There are a number of types of Free Trader, both non-relativistic and relativistic.
A Non-Relativistic Free Trader is basically a standard interplanetary-capable ship (usually fusion, amat or fusion-amat, occasionally conversion drive powered), and limits itself to the Wormhole Nexus, moving from system to system selling and trading wares. Such ships usually range in size and mass from around a dozen to a hundred thousand tonnes. Some five million Non-Relativistic Free Traders operate across Known Space, and the number is slowly increasing as more systems are connected to the Nexus.
Supertrader: These sub-relativistic vessels of up to several million tonnes are very rare, and mostly limited to the Cygexba, STC, MPA, and NoCoNeg volumes. These larger vessels, which may be up to one or two kilometers in length, are limited by the gauge of the wormhole, as well as very high tolls, but can conversely carry far more freight. There are no more than a few thousand Supertraders in operation.
A Relativistic Free Trader is usually a small to medium-sized vessel, Conversion-drive, Drive-sail, or occasionally reactionless drive powered, which is not limited to the Wormhole Nexus. Because these vessels have much higher operating costs, as well as sometimes requiring hyperturing cybernetics and control for the drive system, they are less common than the non-relativistic variant. Even so, there are estimated to be at least several hundred thousand of these ships in service today
Fast Trader is a special type of Relativistic Free Trader capable of very rapid acceleration, and operating at very high relativistic velocities a long way from the nearest wormhole terminus. Fast traders are used primarily to deliver small, precious cargoes - data and so forth - at very high relativistic speeds, often using reactionless or conversion drives to attain extremely high acceleration. There are perhaps fifty thousand fast traders in existence today.
A Lonely Trader is a Relativistic Free Trader operating a long way from the nearest stargate. Such vessels find it very hard to compete with corporate and empire traders, and only because their ship-board hyperturing has some quixotic reason for keeping things going. Perhaps ten thousand throughout known space.
Megatrader. These are large relativistic vessels ranging from several hundred thousand to several tens of millions tonnes, with lengths of up to a kilometer or more, which compete against the big empire and corporate traders. They are of a high tech level (almost always reactionless drive) always directed by one or more on-board hyperturings. Some have sapient crews, mostly for memetic, sometimes for pragmatic, purposes, others do without. There are thought to be almost a hundred thousand in operation.
Hypertrader. A Free Trader with no crew of less than SI:1; sometimes the crew are all ai or po of SI:2. Every hypertrader is different in its configuration and preferred cargo. Almost all hypertradera are based in the FAS, Orion Federation, or the TRHN empires, but from there they range widely throughout the nexus and beyond.