I agree, it probably comes from maths. As a side note, the first letters are used for constants, the last letters are for variables; and it was Descartes who p

I agree, it probably comes from maths. As a side note, the first letters are used for constants, the last letters are for variables; and it was Descartes who popularized this usage: http://en.wikipedia.org/wiki/Polynomial#Notation http://en.wikipedia.org/wiki/Polynomial#Notation I believe they used i for index, and also for the reason that it wasn't used by other usages as above. Similarly, the integration sign is actually a long s, standing for sum. http://en.wikipedia.org/wiki/Integral_symbol http://en.wikipedia.org/wiki/Integral_symbol