I don't know the literal meaning, but it seems to me that this phrase is related to British higher education... e.g. a someone who is 'distinguished in letters' is a person who has received a (doctoral) Degree in Letters (possibly honorary). See here: http://www.lboro.ac.uk/service/publicity/news-releases/2005/62_honorary_degrees.html
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In ordinary everyday letters . . . pi In Greek letters . . . Π
If you mean in 5 letters then it is: TODAY
The mathematical term for "mean" is "mean".The popular, or colloquial term for "mean" is "average".
centimeter
Communication by the exchange of letters