Any theory treating a given subject matter as dependent on human beliefs and attitudes, whether those of an individual, a social group, or humanity generally.
A subjectivist theory of ethics, for example, might analyze an utterance like 'Abortion is wrong' as meaning that the speaker, or his society, or people in general, disapproves of abortion; alternatively it might offer a speech act analysis of it.
Subjectivism is akin to, but not the same as, relativism. The view that 'Abortion is wrong' means that most people disapprove of it is subjectivist but not relationist. The view that jailing an innocent man to prevent a riot is (objectively) right on utilitarianism but wrong on Kantianism and that there is no 'correct' answer, is relativist but not subjectivist.
The contrast term is objectivism.