I think (think!) that in view of "people trade them for other currencies every day on open markets?" it's pretty clear what's meant is "exchange value"[1], though since "value" can be taken a few different ways and it would probably be better to be explicit about which one is intended.

In particular, exchange value or market value can diverge considerably from utility[2], or "use value"[3] - pathologically so in cases like that of the tulip bubble and so on.

[1] http://en.wikipedia.org/wiki/Exchange_value [2] http://en.wikipedia.org/wiki/Utility [3] http://en.wikipedia.org/wiki/Use-value