e is a constant on the TI-83 series calculators that holds the approximate value of Euler's number, fairly important in calculus and other higher-level mathematics.
The approximate value, to as many digits as stored in the calculator, is 2.718281828459…
The main use of e is as the base of the exponential function e^( (which is also a separate function on the calculator), and its inverse, the natural logarithm ln(. From these functions, others such as the trigonometric functions (e.g. sin() and the hyperbolic functions (e.g. sinh() can be defined. In re^θi mode, e is used in an alternate form of expressing complex numbers.
Important as the number e is, nine times out of ten you won't need the constant itself when using your calculator, but rather the e^( and ln( functions.