A dot matrix is a 2-dimensional patterned array, used to represent characters, symbols and images. Every type of modern technology uses dot matrices for display of information, including cell phones, televisions, and printers.
As an impact printer, the term mainly refers to low-resolution impact printers, with a column of 8, 9 or 24 "pins" hitting an ink-impregnated fabric ribbon, like a typewriter ribbon, onto the paper.
However, almost all modern computer printers also create their output as matrices of dots, but use a different technology like laser printing or inkjet printing and are not called dot matrix printers.
Impact printers survive where multi-part forms are needed, as the pins can impress dots through multiple layers of paper to make a carbonless copy, for security purposes.