Question:  I noticed on some of the wireless brochures that antenna gain is referenced as "dBi".  What does the term dBi stand for?
Answer:
dB stands for decibel, which is a way of stating numbers in respect to each other. The “I” stands for isotropic, a theoretical antenna that radiates its signal equally well in all possible directions. The term dB, by itself, has no meaning, it must have a reference. For antennas used in the Wi-Fi band, the reference is the isotropic antenna. For power levels, the reference is the milliwatt (1/1000 of a watt).
An antenna that has 3 dBi gain means that it effectively radiates twice the power put into it as compared to the isotropic antenna. A 3dB increase doubles the output while a 3dB decrease reduces the power by half. Since we live in a real world and don’t get something from nothing, the gain actually comes from compressing the pattern into a narrower area.  Imagine placing a balloon on a table (representing the isotropic antenna) and then taking your hand and pressing down on the top of the balloon. The height of the balloon decreases, but the circumference increases. This is how you get gain out of an antenna. The down side is that you reduce the radiated signal above and below the main signal.