An expression is simply values (variable or constant) and operators. For example:

          y = x
          y = x + y
          y = -x - y
          y = (5 + (3 - 2x)) / z

Please take time to look at the operator and casting sections, as many bug reports have been generated by a misunderstanding.

Like C, but unlike Pascal, variables of different types can be mixed freely in an expression. In this case, the promotion rules listed under "operators" are in effect.