What is "Expression" referring to?

Question

Prefix increments the value, and then proceeds with the "expression".
Postfix evaluates the "expression" and then performs the incrementing.

Answer 1

An expression is a combination of operators and operands in general.
a + b
a++ - b--
a * b - c
++a + ++b
a / b + c

Above all are legal combinations and can be called an expression.