A member of a formally constituted Communist Party.
...all through the between-war years no Socialist programme that was both revolutionary and workable ever appeared; basically, no doubt, because no one genuinely wanted any major change to happen... The Communists wanted to go on and on, suffering a comfortable martyrdom, meeting with endless defeats and afterwards putting the blame on other people.1941, George Orwell, The Lion and the Unicorn, Pt. III:
2
A proponent of the kind of communism formally espoused by such a party, typically either Marxism, or more commonly, Leninism or Maoism.
3
Any member of an organization, especially a military force, controlled and led by a formally constituted Communist Party, even if most individual members of the organization are not party members.
For example, what will happen if a landing is achieved and the Communists are advancing with some success?1951, Memorandum on the Substance of Discussions at a Department of State–Joint Chiefs of Staff Meeting, January 30, 1951:
4
Proscribed, Sometimes Alternative letter-case form of communist, a proponent of the general philosophy of communism.