sem-el-
“once, a single time”simple, simultaneous, similar
Extended form of *sem- giving Latin similis/simplex, English simple, simultaneous, similar, resemble.
Discussion
The Proto-Indo-European root *sem- carried the fundamental meaning "one, together, as one," and from it was derived the form *sem-el- with the adverbial sense "once, at a single time, in one go." Pokorny (IEW 902–905) documents the extraordinary productivity of this root, which generated vocabulary for unity, identity, similarity, and simplicity across every major branch of the Indo-European family. The suffixed form *sem-el- appears most directly in Latin semel, meaning "once," but the root's broader derivatives in Latin are even more consequential for the modern European lexicon. Similis, meaning "like" or "resembling," preserves the logic that things which are one in kind are similar; from it English receives similar, simile, and simulate. Simul, meaning "at the same time," gives English simultaneous — things happening as one. Simplex, literally "one-fold" from *sem- plus *plek- (to fold), yields English simple, and its opposite multiplex follows the same architectural logic. The compound singulus, from *sem- with a diminutive suffix, produced single through Old French. Watkins traces the Germanic reflexes through *samaz, which gives English same, and through *sandiz, yielding the suffix -some as in "handsome." The Greek reflex is equally important: the prefix homo-, as in homos meaning "same" or "one and the same," descends from *somHo-, a zero-grade form of the root, and has become one of the most productive combining forms in scientific and philosophical vocabulary. Beekes confirms this derivation while noting certain phonological complexities in the Greek vocalism. The root also produced Sanskrit sam- and sama- (together, even, equal), visible in saṃskṛta-, the origin of the word Sanskrit itself, literally meaning "put together, well-formed." Few Proto-Indo-European roots have so thoroughly saturated modern discourse: when English speakers say that something is simple, that two things are similar, that events are simultaneous, or that a substance is homogeneous, they are reaching back each time to a single prehistoric syllable meaning "one."