An expression is a formal construct in mathematics and computer languages. A very general definition of an expression is as follows:

Expression : a (complex) formula that returns a value.

Some examples of expressions: 1 + 2 with result: 3 'de groene' + 'boom' with result: 'de groeneboom' x * x + 2 * y result depends on the values of the variables x and y

For more details on the meaning and use of expressions see e.g wikipedia .