sem-
“one, together”one, same, together
Root for oneness/togetherness, yielding Latin simul/semel, English same, some, simple.
Discussion
The PIE root *sem- (one, together, as one) is treated in full at the related entry under a different notation. The root produced: English same (OE same, from PGmc *samaz), Latin simul (at the same time → simultaneous), similis (like → similar, simile), simplex (one-fold → simple), and singulus (single). Greek homos (same) gave the combining form homo- (homogeneous, homophone). Sanskrit sam- and sama- (together, even) continue the Indo-Iranian reflex.
The root's significance lies in its extraordinary productivity: from a single PIE syllable meaning "one" arose the vocabularies of identity (same), similarity (similar), simplicity (simple), simultaneity (simultaneous), and unity (ensemble). When English speakers say that something is simple, that two things are similar, or that events are simultaneous, they reach back each time to this prehistoric syllable.
Notes
Pokorny 902-905. English same, some, simple, simultaneous, assemble, ensemble.