The English noun mass is derived from Middle Latin missa. The Latin word was adopted in Old English as mæsse (via a Vulgar Latin form *messa), and was sometimes glossed as sendnes (i.e. "a sending, dismission").
The Latin term missa itself was in use by the 6th century.
It is most likely derived from the concluding formula Ite, missa est ("Go; the dismissal is made"); missa here is a Late Latin substantive corresponding to classical missio.
Historically, however, there have been other explanations of the noun missa, i.e. as not derived from the formula ite, missa est. Fortescue (1910) cites older, "fanciful" etymological explanations, notably a latinization of Hebrew matzâh (מַצָּה) "unleavened bread; oblation", a derivation favoured in the 16th century by Reuchlin and Luther, or Greek μύησις "initiation", or even Germanic mese "assembly". The French historian Du Cange in 1678 reported "various opinions on the origin" of the noun missa "mass", including the derivation from Hebrew matzah (Missah, id est, oblatio), here attributed to Caesar Baronius. The Hebrew derivation is learned speculation from 16th-century philology; medieval authorities did derive the noun missa from the verb mittere, but not in connection with the formula ite, missa est.
Thus, De divinis officiis (9th century) explains the word as a mittendo, quod nos mittat ad Deo ("from 'sending', that which sends us towards God"),
while Rupert of Deutz (early 12th century) derives it from a "dismissal" of the "enmities which had been between God and men" (inimicitiarum quæ erant inter Deum et homines).