dissident
Concise Oxford English Dictionary © 2008 Oxford University Press:
dissident /ˈdɪsɪd(ə)nt/
▶noun a person who opposes official policy. ▶adjective in opposition to official policy.
– derivatives
dissidence noun.
dissidence noun.
– origin C16: from L. dissident-, dissidere ‘sit apart, disagree’.
'dissident' also found in these Oxford entries:

