Relativism is "The doctrine that no ideas or beliefs are universally true but that all are, instead, “relative” — that is, their validity depends on the circumstances in which they are applied."
One example is moral relativism, which rejects the existence of absolute morality and often argues that morality is a social construct that varies in different cultures. It has been criticized for a variety of reasons, such as implying the absence of universal human rights and has often been used as a reason for rejecting various human rights criticisms.
Evolutionary psychology provides a non-religious explanation for morality, as genetically influenced instincts, that were/are evolutionary beneficial (in particular for a genetically related group). In many cases, these instincts are universal and present in all societies (with the exception of psychopaths), although such genetic instincts may to some degree by suppressed by, for example, Cultural Marxist propaganda. However, evolutionary psychology is not a very popular explanation for traditional morality, since religious conservatives often dislike evolutionary explanations and liberals often dislike traditional morality.
Cultural relativism is the principle that an individual's beliefs and activities should be understood by others in terms of that individual's own culture. It is a common principle in modern Boasian anthropology. It is sometimes argued to not imply moral relativism, but this is a contested and debated issue.
Subjectivism may be seen as a form a relativism, in which the emphasis is on ideas and beliefs as relative to an individual's subjective experience, rather than as relative to culture. The corresponding form of moral relativism is ethical subjectivism.
More generally, it is sometimes argued that there are no absolute truths, i.e., that truth is always relative to some particular frame of reference, such as a language.
Another tactic is extreme skepticism or nihilism. This in particular against disliked views, to which an extremely stringent double standard may be applied. The disliked view is then argued to lack sufficient evidence, and if influential, to be so due to being useful relative to the goals of those with power. Such arguments may also be applied to knowledge in general.
Such views and such tactics against disliked views have been very influential in humanities and social sciences and are associated with Cultural Marxism.
Advocates of such relativism (as well as various "New Age" mystics) may even claim that developments in the natural sciences, such as the uncertainty principle in quantum mechanics, shows that even natural science is now becoming relativistic. This has however been rejected as dishonest quackery. "Modern physics, including quantum mechanics, remains completely materialistic and reductionistic while being consistent with all scientific observations. [...] Furthermore, interpretations of quantum effects need not so uproot classical physics, or common sense, as to render them inoperable on all scales-especially the macroscopic scale on which humans function. Newtonian physics, which successfully describes virtually all macroscopic phenomena, follows smoothly as the many-particle limit of quantum mechanics."
Philosophical/scientific objectivism is in principle the opposite of philosophical/scientific relativism/subjectivism. Objectivists usually do not deny that cultural and individual factors may have influential effects, but do no see such factors as fundamental. However, the term objectivism has been conflated with Randian Objectivism.
Related philosophical discussions are regarding the existence of a mind-independent reality and whether is it possible to measure this reality - realism vs. various other concepts. Different authors do not necessarily agree on the meanings and relations of these terms.