Complete each statement. The following key terms may be used once, more than once, or not at all.
$$\begin{array}{cc}\text{linear equation}&\text{solution}&\text{algebraic expression}&\text{contradiction}& \text{all real numbers}\\ \text{solution set}&\text{identity}&\text{conditional equation}&\text{first-degree equation }&\text{empty set}\varnothing
\end{array}$$
A collection of numbers, variables, operation symbols, and grouping symbols, such as $2(8 x-15),$ is a $(n)$ __________. While an equation (does/does not) include an equality symbol, there (is / is not) an equality symbol in an algebraic expression.