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The AB system of magnitudes (or AB magnitude) sets the zero point for monochromatic magnitudes, specifically setting it so that zero represents 3631 janskys. Though the system is explained as monochromatic, this merely means it is calculated for a band by dividing by the EMR energy received by the frequency-interval (bandwidth), which is how janskys are calculated. Such a measure can only be determined from data over a non-zero bandwidth, i.e., no dividing by zero. The AB magnitude is defined as:
mAB = - 5/2 log10( fν/3631 )
Magnitudes require some adopted zero point as a consequence of their being logarithmic: "zero flux" cannot be used since zero has no logarithm (or its logarithm might be referred to as "minus infinity"). A brightness must be chosen to be represented by "zero magnitude", part of the definition of the magnitude's scale.
AB stands for "absolute". Among other methods of calibrating magnitudes is the Vega system, designed around using a specific star (Vega) to peg at zero magnitude (thus, individual observations could be calibrated by viewing the source-of-interest and also viewing Vega and comparing). Such systems are used with passbands: for example, to use the AB system, the formula above is used with the passband's sensitivity function. Converting between the AB system and systems such as the Vega system for the same passband can be done through use of Vega's magnitudes in the AB system. To additionally convert between differing passbands, the passbands' sensitivity-functions also affect the relation, and I believe also the SED of the source. There are simple formulae for approximate conversions which I believe assume the typical SED of stars.
Example conversion of passbands (UBV photometric system), from the Vega system to the AB system, with Sirius A as an example star (I don't think using the AB system with UBV is common: this is just an example):
| Passband | Vega system zero point in janskys | to convert Vega to AB | Sirius(Vega) | Sirius(AB) |
| U | 1823 | -0.748 | -1.51 | -2.26 |
| B | 4130 | +0.140 | -1.46 | -0.06 |
| V | 3781 | +0.044 | -1.46 | -1.416 |
(Cited zero points in janskys for the Vega system for these passbands vary a little bit.)
The term maggy or maggie is a colloquial abbreviation of magnitude, seemingly most often used for AB magnitudes, i.e., "2.1 maggies".