noun
Usage: often capitalized M
: a mass celebrated in place of the mass appointed for the day except when that takes precedence due to festal rank or identity of intention and offered in special devotion (as in honor of a saint or the angels) or applied for the benefit of a particular purpose (as peace or the spread of the faith) or person (as an invalid) : a mass provided in the missal for a special intention, occasion, or devotion:
a. : one of a strict class that must be said on a certain day including masses (as a mass of the Blessed Virgin on Saturdays or a requiem on All Souls' Day) prescribed in the rubrics of the missal, solemn masses ordered by the pope or ordinary for grave occasions (as the election of a bishop or a time of war), masses of the forty hours devotion, and nuptial and some requiem masses
b. : one (as for the private intention of a donor) that may be said at the discretion of the priest