How is coords.accuracy measured, what unit is it in?

It is typically expressed in meters.

The Geolocation API specified by W3C says:

The accuracy attribute denotes the accuracy level of the latitude and longitude coordinates. It is specified in meters and must be supported by all implementations. The value of the accuracy attribute must be a non-negative real number.

It's meters for PhoneGap, Google's various geo APIs, and so on. It's almost certain that meters are the unit of measurement in whatever you're using.