Definition of XOR, and how to remember its expansion in disjunctive normal form for N variables
Although it is common to use the schematic symbol for xor to describe gates with arbitrary numbers of inputs whose output is true when an odd number of inputs are true, and xnor for one whose output is true when an even number of inputs are true, the notion that the terms "xor" and "xnor" should refer to such gates with more than two inputs is not quite universally accepted.  Some implementations define xor as being true when some but not all of the inputs are true; others define it as being true only when exactly one input is true.  Because there isn't a 100% consensus on what a three-input xor means, it's probably best to avoid using such gates in any context where the meaning wouldn't be apparent.