In mathematical terms, the definition of a proportion is:

A proportion is defined as a statement that expresses the equality of two ratios. In mathematical terms, this means that if you have two fractions, a/b and c/d, a proportion states that a/b = c/d. This relationship demonstrates that the two ratios are equivalent and helps in situations where you need to determine a missing value based on the given ratios.

The essence of proportions lies in their ability to show a balance between two ratios, which is critical in various applications like solving for unknowns in word problems, scaling recipes, or converting measurements. Understanding proportions is fundamental for students because they form the basis of many mathematical concepts and real-world scenarios.

The other options do not capture this crucial aspect of proportions. While statements of equality might describe a different mathematical concept unrelated to ratios, expressions involving operations or algebraic expressions do not necessarily imply a comparison between two equal ratios. Comparisons of different sets of data might involve proportions, but they aren't inherently defined by this comparison alone.