It is absolutely fine to do so. 

Look at [this][1]. See how many authors/books have already used it. 


  [1]: https://www.google.com/search?q=%22matrix%20of%20order%20n%20by%20m%22&rlz=1C1CHBD_en&biw=1920&bih=937&tbm=bks&sxsrf=AOaemvJQsBxMJnFsYE4pS_TEWssolGbxqQ%3A1631812529132&ei=sXtDYa3KB_q_3LUP5ua7mAI&oq=%22matrix%20of%20order%20n%20by%20m%22&gs_l=psy-ab.3...2396.5609.0.5766.2.2.0.0.0.0.200.200.2-1.1.0....0...1c.1.64.psy-ab..1.0.0....0._FcmDsGvvpc