Origins
The word 'sequel' means 'that which follows,' and its etymology is a straightforward expression of this meaning. Latin 'sequēla' was a noun derived from 'sequī' (to follow), and it described anything that came after or as a result of something else — a consequence, a continuation, a next stage. English borrowed it through Old French in the early fifteenth century, and it has retained this core meaning through six centuries of use.
Latin 'sequī' (to follow) descends from the Proto-Indo-European root *sekʷ-, meaning 'to follow.' This root generated one of the largest and most semantically diverse word families in English, all unified by the concept of following.
The direct derivatives are the most obvious: 'sequence' (a following of one thing after another), 'subsequent' (following after), 'consequence' (a following together — what follows from an action), and 'sequel' itself. But the family extends much further.
Proto-Indo-European Roots
'Second' comes from Latin 'secundus,' which literally means 'following' — the second item is the one that follows the first. 'Sect' (from Latin 'secta,' a following, a school of thought) describes a group that follows a particular leader or doctrine. 'Pursue' (from Old French 'poursuir,' from Vulgar Latin *prosequere) means to follow after. 'Prosecute' (from Latin 'prōsequī') means to follow through with a legal action. 'Execute' (from Latin 'exsequī,' to follow out) means to carry out or follow through to completion. 'Sue' (from Old French 'sivre/suir,' from Latin 'sequī') originally meant simply 'to follow' and later narrowed to 'to follow into court.' 'Suit' and 'suite' (a following set) are the same word in different spellings.
Even 'segue' — the smooth transition from one thing to another — comes from Italian 'segue' (it follows), the third person singular of 'seguire' (to follow), from Latin 'sequī.' The musical and broadcasting term literally means 'it follows' — indicating that the next section follows without interruption.
The entertainment sense of 'sequel' — a movie, book, or other work that continues the story of a previous one — became dominant in the twentieth century, particularly with the rise of Hollywood franchise filmmaking. 'The Godfather Part II' (1974) is often cited as the film that proved sequels could be artistically successful, not merely commercially exploitative. The proliferation of sequels, prequels (a back-formation: what comes before), and 'cinematic universes' has made 'sequel' one of the most commonly used words in entertainment journalism.
Latin Roots
Before its entertainment dominance, 'sequel' was used more broadly for any consequence or result. A disease might have 'sequels' (aftereffects — the medical term 'sequelae' preserves this sense in its Latin plural form). A war might have diplomatic sequels. An argument might have personal sequels. This broader usage persists in formal writing but has been largely overshadowed by the entertainment meaning in everyday speech.
The word 'prequel' — denoting a work set before the events of an existing work — was coined in 1958 (first attested in print in that year), formed by analogy with 'sequel' by replacing 'se-' with 'pre-.' It is a pseudo-Latin formation: Latin would not have constructed the word this way, since 'se-' in 'sequel' is not actually a separable prefix (it is part of the verb stem 'sequī'). Nevertheless, the back-formation is universally understood and widely used.
From PIE *sekʷ- through Latin 'sequī' to modern 'sequel,' the word traces one of language's most fundamental concepts: the relationship between what comes before and what comes after. Every sequel, in every sense, is defined by following — by its relationship to what preceded it.