According to Dr. Laura Robinson, DVM, the term “arthritis” is a catch-all term for any disease that causes inflammation and stiffness of the joints. People can get rheumatoid arthritis or osteoarthritis.
In the veterinary community, people shorten the term and use “arthritis,” but are generally referring to osteoarthritis. Osteoarthritis is a loss of cartilage in the joints, which causes inflammation and makes movement painful. It’s a progressive degenerative disease of the joints and it actually affects about 20% of dogs of all ages, so it’s pretty prevalent.