In math, a unary operation is an operation with a single input. In Ruby, a unary operator is an operator which only takes a single ‘argument’ in the form of a receiver. For example, the – on -5 or ! on !true.
In contrast, a binary operator, such as in 2 + 3, deals with two arguments. Here, 2 and 3 (which become one receiver and one argument in a method call to +).
Ruby only has a handful of unary operators, and while it’s common to redefine binary operators like + or  to give your objects some added syntactic sugar, unary operators are less commonly redefined. Read More