Why maximum a posterior, not maximum posterior? Is the additional "a" mean that different priors may lead to different posterior, MAP is a result of many possible results?  And similar to MLE, why the abbreviation of maximum a posterior estimation is MAP, not MPE or MAPE? Thanks

The phrase a posteriori is from Latin, the sort of Latin phrase that used to appear in educated British English.  Along with a priori, it was already in use in philosophy/logic in a slightly different sense before it got used in statistics.  There's not really any significance to the a, just history.
