- sine qua non: Late Latin; literally "without which not"; meaning something essential. As a mathematician, I wanted to pronounce "sine" as in "sine and cosine", but I know that in Latin that arrangement of vowels and consonants should be two syllables. I'll invent my own typology here to avoid diacritical marks-- "i" is short as in "tip"; "a" is as the "o" in "bother" or "cot": si' ni qua nan' preferred by Webster's but a long I as in sigh' ni qua non is also used.
- ne plus ultra: New Latin (OK, next question--what's difference between Late Latin and New Latin? some other time, perhaps) for "no more beyond"; meaning the highest point capable of being attained or the most profound degree of a quality. Use "u" as in both the a and u of abut and A as in "day" or "fade": nA plus ultru.
In any event, I was glad to confirm that I had picked the right one and had pronounced it reasonably close to the correct manner. Another vocabulary faux pas avoided!