In a sentence like PvQ, we said that P and Q are the inputs, and the complex sentence PvQ is the output.
The number of inputs a connective has is called its arity.
If a connective takes one input, it is called unary. If it takes two inputs, it is called binary.
"Arity" is just a funny word made from the suffix of these words to stand for the general concept.
Binary: a connective with two inputs.
You can have connectives with three or more inputs too, which we will discuss in a later chapter.
Conjunction and disjunction connect two sentences together to make a complex sentence, so they are binary. (Remember, technically v and & connect two sentences, even though we allow a long string of &s or a long string of "v"s.)
Negation is unary, but remember that we still call it a connective!
A connective's arity is fixed: ~ is always unary; & and v are always binary. (Even in P&Q&R, we call & binary.)