What speakers mean by it
In present-day English a reunion is two closely related things. It is the act, or the resulting state, of coming together again after a split. It is also a planned gathering of people who once formed a group and then scattered: relatives, classmates, veterans, a band. The second sense is the one that fills invitations and alumni mail. Both rest on the same idea, a union that happens another time.
The origin of the word reunion is unusually easy to see on the page. English speakers can still cut it into the prefix re- and the noun union. That reading is historically genuine. It is not, however, a complete account of how the noun entered the language, or of why a high-school dinner and a political rejoining share one label.
A French noun, and parts English already had
Reference works do not tell a single story about the last hop into English. The Oxford English Dictionary classifies reunion as a borrowing from French reunion, later spelled with the accent as réunion. Merriam-Webster dates the English noun to 1576. Other desk dictionaries round first use to about 1600–1610 and describe the word as English re- attached to union, which English already possessed. Etymological handbooks often list both routes: a French loan, or a combination built from pieces already on hand.
Those accounts can sit together. French had formed réunion from the verb réunir on the model of union well before English writers needed the noun. Middle French already used it in the later fifteenth century for reconciliation and for the act of joining again, including the folding of territory into a realm. English, for its part, had union from the early 1400s, by way of Anglo-French and Old French, and had treated re- as a living prefix for centuries. A late-Tudor translator could have taken the French noun, or assembled the same pieces at home. The result would look the same on the page.
The earliest firm English evidence is from 1576, in a translation by Arthur Golding. That is still the abstract act of joining again, not the alumni party the word now suggests. The related verb reunite is a little older in English, from around 1500, and tracks Medieval Latin reunire, built from re- plus Late Latin unire, to make one. Collins traces the English noun through French réunion to Medieval Latin reunio. Some New Oxford notes also allow an Anglo-Latin form reūniō. The last step into English is therefore slightly uncertain. The Latin and French family behind it is not.
Again, and one
The inner noun is union. English took it in the early fifteenth century as unioun, covering the act of joining things into one, agreement, and marriage. Behind that stand Old French union and Late Latin unio, unionem, oneness or a uniting. Classical Latin also used unio for a single large pearl and, by a joke of layered shape, an onion. The base is unus, one, from an Indo-European root reconstructed as *oi-no-, one or unique.
The prefix is Latin re-, which English received both from Old French and directly from Latin. Its core senses are back and again. On this noun it is the second of those: not an undoing, but a repeat of joining. The morphology is therefore real, not a folk tale pasted onto an opaque loan. A speaker who hears re- plus union is making the same cut French speakers made when they derived réunion from réunir beside union.
Close relatives keep that shape across Romance. French réunion, Spanish reunión, and Portuguese reunião are near kin. Romanian reuniune is a later descendant of the French noun. Dutch reünie wears a Germanic spelling of the same stem. German Reunion has been borrowed more than once: first as a term for French territorial policy in the early modern period, and later, through English, for a group that meets again, including a band returning to the stage. These are not chance lookalikes. They share the Latin habit of placing again in front of one.
From an act to a date on the calendar
The first English sense is abstract: the act of coming together again. That use is still alive when families meet after a war, or when political bodies join after a rupture. A more personal sense, people meeting again after a separation, is recorded from the early eighteenth century. The familiar social event, a gathering of people who already share a connection, is later still, from about 1820. In that use English sometimes kept a French look and wrote réunion into the 1830s, a hint that the party sense still felt slightly imported.
French had already moved along a similar path, though not on the same timetable. By the 1460s réunion could mean reconciliation. By 1477 it could mean annexation. The sense of an assembly of persons is clear in French by the 1790s. English did not really adopt the old French legal sense of folding a province back into a crown. What it did take, and then specialize, is the human gathering.
That specialization is why a class reunion is not merely any second meeting. It assumes a prior group, a stretch of absence, and a planned return. The word still names the abstract rejoining, but for many speakers the countable event has become the default.
A place-name that still means joining
One proper name keeps the older political flavor in view. The Indian Ocean island now called Réunion was Bourbon under the French crown. In 1793, during the Revolution, it was renamed to mark the 1792 joining of Marseilles revolutionaries with the National Guard in Paris. The Bourbon name returned after 1815, then the Revolutionary name again after 1848. English reunion and the island name are doublets: the same French noun, one as a common word, one as geography.
The 1576 date in Golding is the useful remainder. Long before printed invitations, the English noun simply named the fact of becoming one again.