A mate, companion, or associate.
A companion in battle; fellow soldier.
A fellow socialist, communist or other similarly politically aligned person.
Hello, comrade. Are you going to the Communist Party meeting tonight?
A gender-neutral title, functionally similar to "Mr.", "Mrs.", "Miss", "Ms." etc, in a communist or socialist state.
Comrade Lenin inspired our people to undertake great works.
To associate with in a friendly way.