One of the mysteries of the English language finally explained.
An emission line in the spectrum of an element which is the last (or one of the last) to remain detectable as the concentration of that element decreases.
1920s. From French raie ultime from raie line, stripe, band (in a spectrum) + ultime ultimate, last.
raie ultime/ˌreɪ ʊlˈtiːm/
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