Simplify the following text for a Grade 9 native English-speaking person.
Original: To specify the size of a matrix , a matrix with m rows and n columns is called an m-by-n matrix or m Ã -- n matrix , while m and n are called its dimensions .
Simplified: A matrix with m rows and n columns is called an m-by-n matrix ( or mÃ -- n matrix ) and m and n are called its dimensions .