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I see it all the time, it's important and I have no idea what it is!!
This is a quick and superficial overview of the deciBel (or colloqually the dB) (see the side bar links for the real 'stuff').
The deciBel is a unitless relative measure of power between two values and is most commonly used to describe gain or loss in wireless (and other) communications systems. The standard 'power ratio' equation for dB is written as:
dB = 10 log _{10} ( P _{2} / P _{1})
In the above equation P_{1} is the reference value and in most wireless systems this is 1 milliWatt (mW), so dB strictly speaking should be written as dBm to indicate the reference value being used. Finally, to precisely define the value a resistance is also assumed which for most wireless systems is 50 ohms so the final form should be dBm(50). You may also see other forms of db the most common of which are defined in this table.
You can calculate the value you want by substituting the P_{2} for the power of your radio and P_{1} for 1 mW, use our conversion calculator, one of the excellent dB calculators referenced in the links section, or you can read the value from the table below!
Remember: The dB is a log not a number, so a difference of 10 dB is a FACTOR of 10, 3 dB is almost twice the power, and so on.
Negative dBm (for example, 87 dBm) just says that the power is less than 1 milliwatt and is most frequently used to describe Receiver Sensitivity. Typical receivers will operate in the range 80 to 110 dBm. With typical free space loss being over a 100 dBm for just sending down the corridor  you need it.
dBm  Watts  Notes  dBm  Watts  Notes 
0  1.0 mW  19  79 mW  
1  1.3 mW  20  100 mW  
2  1.5 mW  21  126 mW  
3  2.0 mW  22  158 mW  
4  2.5 mW  23  200 mW  
5  3.2 mW  24  250 mW  
6  4.0 mW  25  316 mW  
7  5.0 mW  26  398 mW  
8  6.3 mW  27  500 mW  
9  8.0 mW  28  630 mW  
10  10.0 mW  29  800 mW  
11  12.5 mW  30  1.0 W  Maximum radio power for FHSS in 2.4 GHz ISM band  
12  15.8 mW  31  1.3 W  
13  20 mW  32  1.6 W  
14  25 mW  33  2.0 W  
15  32 mW  34  2.5 W  
16  40 mW  35  3.2 W  
17  1.0mW  36  4.0 W  Maximum EIRP for FHSS in 2.4 GHz ISM band.  
18  63 mW  37  5.0 W 
Term  Meaning 
dB  Completely meaningless since it defines no base (it should be something like dBm or dBW, see table below) but depending on context usually means the 'most common measure', for example, in low power wireless it is normally the power ratio relative to 1 milliWatt and therefore should be written as dBm. 
dBi  Some times used to describe antenna gain the 'i' stands for 'isotropic' 
dBm  The power ratio relative to 1 milliWatt 
dBW  The power ration relative to 1 Watt 
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