What is the math symbol i?

Answer 1

The most common uses of 'i' follow, however there are many and clarifying your question with context may allow a more useful answer:

'i' may be used as a general case when items are to be counted, e.g. "the ith member of a set", or in a summation where the iterating variable is commonly given as 'i'.

'i' may also be used as the unit vector in the direction of the first axis (commonly denoted 'x'), in which case it may be given in bold text or with decoration to denote it signifies a vector.

Finally, 'i' may be used to denote the imaginary part of a complex (or wholly imaginary) number, for example, C = 4 + 3i is a complex number with:

Re{C} = 4; Im{C} = 3