A matrix that is not a square matrix, is called a______ matrix. (Fill in the blank.)

Hint: When we talk about a matrix, then it is known as a rectangular sequence of numbers, or alphabets, or some expressions which can be arranged in certain rows and columns. In plural form, matrices are known as matrices.

Complete step by step answer:
A matrix is always represented as a pair of numbers, or alphabets, etc, . inside brackets like \[\left[ {\begin{array}{*{20}{c}}
  {{a_{11}}}&{{a_{12}}} \\
  {{a_{21}}}&{{a_{22}}}
\end{array}} \right]\], where the alphabets inside the brackets represent something.
First, we should know what a square matrix is. A square matrix is a matrix that contains the same number of rows and the same number of columns.
Then, we should know the order of a matrix. An order of matrices refers to the number of rows and columns it has. Based on the orders, matrices can be
If a matrix is not a square matrix, then it is known as a rectangular matrix. We can also say that the matrices which have different numbers of rows and columns are called rectangular matrices.
For example, \[\left[ {\begin{array}{*{20}{c}}
  5&9 \\
  3&8
\end{array}} \right]\]is a square matrix. As we see, it has the same number of rows and columns
But, if we look at\[\left[ {\begin{array}{*{20}{c}}
  3&4 \\
  8&6 \\
  8&4
\end{array}} \right]\], then we can say that it is a rectangular matrix. Here, the number of rows and columns are different.
Therefore, a matrix which is not a square matrix is called a rectangular matrix.

Note: The matrix which is not a square matrix is not always a rectangular matrix. There are other types of matrices also present like row matrices, column matrices, diagonal matrices are also there. These matrices can be useful for writing and functioning with multiple linear equations, or to a system of linear equations.