tatanab is right about common parlance, although marketing literature might be a little more precise. Essentially, what's not said is significant.
Single speed means just that - there is only one gear, which could strictly be either fixed or freewheel. We used to talk, for example, about a "10-speed" racing bike, which literally had 10 gears available in total (using a freewheel). But if a bike is said to be single-speed and no fixed wheel is mentioned, a single-speed freewheel is being assumed.
Fixed wheels were universal before freewheel mechanisms were invented. Essentially, the sprocket (round which the chain runs) is just a piece of metal bolted to the wheel hub. So, when the wheel turns, so does the chain and therefore the pedals and, as tatanab says, you can't stop pedalling as long as the bike is moving. With some very, very rare exceptions, fixed wheel bikes are single-speed - it's tricky to make mechanisms to change gear and (almost) no-one does. Fixed is comparatively rare now, but some of us like it and still ride it (and couriers in cities also have something of a cult around it).
If you mention a freewheel, in the absence of any other information, and don't say that it's single-speed, you probably mean a multiple-sprocket freewheel that screws onto the wheel, which is how sports/racing and many touring bikes were done until the late 20th Century. Just to complicate it, there are now freehubs; in essence, that's the same as a freewheel, but the actual freewheel mechanism is part of the hub and, as a general statement, is a stronger way of building things and far more common now.
Marketing literature is always likely to mention how many gears/speeds there are. If it's more than one, there's no chance of a fixed wheel. If it mentions a freehub as distinct from a freewheel, there are definitely multiple gears, and the manufacturer has probably used slightly better parts, but it's not a show-stopper. If it only talks about a freewheel and gives no evidence of multiple gears, there may be only one. But when cyclists talk about freewheels in isolation, we normally mean ones with multiple sprockets (gears).
tatanab's version was simpler.