In some languages other than English, one distinguishes between 'variables' in functions and 'unknown quantities' in equations ('incógnita' in Portuguese/Spanish, 'inconnue' in French) but this does not completely clarify the situation. The terms such as 'variable' and 'parameter' cannot be precisely defined at this stage and are best left to be introduced later in the development of algebra.
An algebraic expression is an expression involving numbers, parentheses, operation signs and pronumerals that becomes a number when numbers are substituted for the pronumerals. For example 2x + 5 is an expression but +) × is not.

Examples of algebraic expressions are:

\begin{align}3x + 1 \hspace{10mm}\text{and}\ \hspace{10mm}5(x^2 + 3x)\end{align}

The multiplication sign is omitted between letters and between a number and a letter. Thus substituting x = 2 gives:

\begin{align}3x + 1 = 3 × 2 + 1 = 7 \hspace{10mm}\text{and}\hspace{10mm} 5(x^2 + 3x) = 5(2^2 + 3 × 2) = 50.\end{align}

In this unit, the emphasis is on expressions, and on the connection to the arithmetic that students have already met with whole numbers and fractions. The values of the pronumerals will therefore be restricted to the whole numbers and non-negative fractions.
In algebra, pronumerals are used to stand for numbers. For example, if a box contains x stones and you put in five more stones, then there are x + 5 stones in the box. You may or may not know what the value of x is (although in this example we do know that x is a whole number).