Term used by Continental philosophers to refer to any system or situation that ticks them off. Hegel is likely at the headwaters of alienation talk. Given his opacity and grandiosity, this explains why many are alienated from his dialectics of alienation. Roughly, Hegel meant that when A is alienated from B, this generates a conflict wherein A gets upset with B. Then, somehow A, prevails over B on the way a mystical ascent into another alienation. But somehow things are getting better, Hegel assures us on the basis of his alienated and dialectically delicious mind. The antagonisms of these alienations form the dialectic of ideas. And so it goes. You get the idea—perhaps. But let us bring it down to earth.
In Marxism (which is more an ideology than a philosophy), claims that workers are alienated from the fruit of their labor through wage slavery. The capitalists supposedly benefit unjustly. The Marxists answer to these perceived maladies resulted in well over 100,000,000 murders in Marxist regimes in the 20th century. These dead workers, I am sure, would rather be alienated from the fruit of the labor than alienated from their earthy existence by authoritarians raving about false philosophies.