Which mathematical concept is described as square or rectangular arrays of numbers?

The concept referred to in the question involves square or rectangular arrays of numbers, which directly relates to matrices. A matrix is structured in rows and columns, allowing for a variety of mathematical operations, including addition, subtraction, and multiplication. Each element in a matrix can be identified by its position, typically expressed as (a_{ij}), where (i) indicates the row and (j) indicates the column.

Matrices are fundamental in various fields, including computer science, engineering, and physics, where they are used for solving systems of equations, transforming geometric spaces, and more. Their structure allows for organized data representation and manipulation, which is crucial in many theoretical and applied contexts.

The other concepts do not match the description provided in the question. Vectors, for instance, are one-dimensional arrays of numbers and lack the two-dimensional structure necessary for the designation of being a matrix. Equations represent mathematical statements rather than a structured arrangement of numbers. Data sets refer more broadly to collections of data points or measurements without implying a specific arrangement, such as that found in matrices. Thus, matrices are the correct and best fit for the description of square or rectangular arrays of numbers.