semel

Semel is an indeclinable numeral adverb and the head of a series: semel, bis, ter, quater, quinquiens. The first syllable is the sem- of simul, similis and English same and single, so the sense underneath is "at one and the same time".

Two idioms are worth having ready. Semel atque iterum, "once and again", means repeatedly, not twice. And semel on its own often carries the force of "once and for all", as in ut semel dicam, "to put it once for all" - a signal that the speaker is closing the subject rather than counting. The one thing to guard against is treating semel as an adjective. "A single X" is unus or singuli; semel never declines, so any ending you think you see on it belongs to the next word.