The four fixed signs of the Zodiac are:
Fixed Signs are associated with stabilization, determination, depth, and persistence. They are powerful and willful in all they do, often achieving much more than the other two qualities. On the other hand, they can also be inflexible, rigid, stubborn, opinionated and single-minded. These traits are often paired with the need to be “correct”: they will ruthlessly fight on behalf of their beliefs, regardless of any contrary beliefs deemed sufficiently credible. Only during moments of importance or necessity would they consider changing an opinion, usually with only half-success. (In loose social terms, these signs tend to “not like admitting that they’re wrong.”)
Definition courtesy Wikipedia.