How are good, fair, poor and bad defined for signal strength? How are good, fair, poor and bad defined for signal strength in terms of dBm, in most Android phones or telecommunication community?

Are they defined the same for mobile network and for WiFi network?

Thanks.
I can answer this for WiFi:  received signal strength indicator (RSSI) in the 802.11 standard is only required to be orderable. This means for a single device you can tell when its reported value x is greater than its reported value y, but not by how much because there may be no real scale. Every device can report different values for the same real signal power, and the values across devices are not directly comparable because they can each use different values and they may have no scales. A device is only required to be able to tell a signal is stronger or weaker than another signal. 

For reference, see
https://en.wikipedia.org/wiki/Received_signal_strength_indication