Originally Posted by zbicyclist

At any given moment in time, any bicycle is going at one speed.
This is true regardless of whether we define "speed" as actual physical motion or as a gear ratio.

Hence, they are all singlespeeds if we fix time.

If we allow time to vary, we can clearly see that any bicycle can have its gear ratio changed, or can move at a different speed than it was moving at some other time. This might be easily done with a grip shift and a derailleur, or done with greater difficulty by replacing the rear wheel or front chainwheel. Hence, allowing time to vary, they are all multispeeds regardless of whether we define "speed" as actual physical motion or as a gear ratio. It's only a matter of the degree of difficulty.

In the face of the semantic and pedantic madness I've just foisted on the group, we need an operational definition. I'd suggest this one: while a non-handicapped person is in motion riding the bike, can this person change the gear ratio? If so, this is a multispeed bike. If not, it is a single speed bike.

By this definition, a multispeed bike with broken derailleur cables becomes a single speed bike, since it cannot be shifted. I can live with that.