ifconfig output for 802.11

[Lars Hansson]

I'm trying to understand the output of ifconfig for 802.11 interfaces 
and while most of it is obvious one part isn't (at least not to me).
In the below output, what exactly does the "20dB" stand for and how do I 
interpret it? Looking at the ifconfig source it would also appear that 
it (whatever it is) could also be expressed as a percentage?

$ ifconfig ural0
......
        ieee80211: nwid unet chan 6 bssid 00:0f:3d:0d:eb:ac 20dB nwkey 
0x8353c823e7 100dBm

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Lars Hansson