The meaning of "city" and "highway" driving here at Fuelly is really up to you. It's just way for you to keep track of different types of driving to see if they affect your fuel economy. You can decide what each of those terms mean to you and apply them however you'd like.
For the EPA and their estimates, "city" and "highway" do have a precise meaning. You can see their definition of the terms
here under the section
How are Vehicles Tested?. Here's how the EPA describes their city test:
"The city test is approximately 11 miles long and simulates a stop and go trip with an average speed of about 20 miles per hour (mph). The trip lasts 31 minutes and has 23 stops. About 18 percent of the time is spent idling (as in waiting for traffic lights)."