Identity is a term whose meanings have shifted significantly since the second edition OED entry was written. It derives from the Latin idem ‘same’ and has a substantial technical philosophical history, densely indicated for example in Vocabulaire Européen des Philosophies. The term begins to take on keyword force in British empirical philosophy: in discussion of “personal identity” in Locke (1690), followed by Hume. Once the unity of the soul no longer carries a metaphysical guarantee, how can I know that I am the “same” when the world around me changes? Locke, quoted in OED, puts the issue as follows:
The Identity of the same Man consists..in nothing but a participation of the same continued Life, by constantly fleeting Particles of Matter, in succession vitally united to the same organized Body. Ibid. §9 Consciousness always accompanies thinking,..in this alone consists personal Identity, i.e. the Sameness of a rational Being.
The most interesting uses of identity move from sameness to selfness. Indeed, many of the places in American pragmatist philosophy where readers now believe the concept of identity is at issue are terminologically cast in terms of ‘the self’, as for example in the work of William James and George Herbert Mead.
In recent usage, identity is frequently ambivalent along the axis of necessity and freedom: the term is used to name both what you can't help being but also what you choose to become. This saturation of the spectrum helps give the term appeal. Identity has nevertheless undergone transformation and reversal within this spread of meaning. Its fundamental sense is ‘sameness’, but the word is nowadays understood within a discourse of difference, so much so that some earlier usages seem hardly recognizable. An OED citation from South Africa in the 1920s shows an understanding of what we call identity politics (a term not recorded as a phrase in the current OED entry) absolutely opposite to what we would expect. The writer contrasts the “policy of subordination” and the “policy of identity”. The policy of identity is faulted because it “refuses to acknowledge any real difference between Europeans and natives” – that is, it treats them as the same. Contemporary identity politics would insist on difference.
Identity entered into its great 20c career through psychoanalysis (via Freud's Group Psychology and the Analysis of the Ego [1921, translated 1922)]). However, Freud’s German is identifizierung (a process-noun, derived from a verb); only once in the collected works does he use Identität—suggestively, in a context of Jewish fraternalism. Ego-psychology developed the term as a status-noun in its most influential form in the work of Erik Erikson. Before Erikson, however, Kenneth Burke was the first English-language intellectual to make extended, crucial use of the terms identity and identification, for example in his 1937 Attitudes toward History:
All the issues with which we have been concerned come to a head in the problem of identity. Bourgeois naturalism in its most naive manifestation made a blunt distinction between “individual” and “environment,” hence leading automatically to the notion that an individual's “identity” is something private, peculiar to himself. And when bourgeois psychologists began to discover the falsity of this notion, they still believed in it so thoroughly that they considered all collective aspects of identity under the head of pathology and illusion.
Contemporaneous with Burke, an anthropological conception of culture began to achieve wide currency (cf. Ruth Benedict, Patterns of Culture ); and after World War II, the historian David Potter saw both a challenge and opportunity for history in what he recognized as the emergence of “the term ‘culture’ in a new sense” (People of Plenty, 1954). The primacy of great-power politics had yielded to a democratic need to include “the total experience of a people”; but the only familiar non-political category for defining a people, that of ‘race’, had been wholly discredited by Hitler, and yet the “compulsions of the medium” of narrative historiography required a “unifying device” by which historians could make their subjects cohere. Potter concluded that, “the concept of culture has given to the study of society the same integrating effect which the Freudian concept of the personality has given to the study of the individual,” and to rival these, he proposed a conception of “national character.”
Erikson filled the equivalent gap with the term identity. He put “national characters” in scare quotes and explained that he preferred “to call them . . . national identities” (Childhood and Society, 1950). A telling example of the impact of Erikson's work is that, in Edward Said's first published piece on Palestinian politics and culture (1969), Said quotes almost a page from Erikson’s Young Man Luther (1958) on “identity crisis” in order to help characterize the then current moment in “Palestinian experience.”
Erikson's notion of identity was first formulated as what he called “ego-identity”. He claimed that “psychosocial identity develops out of a gradual integration of all identifications. . . . [H]ere, if anywhere, the whole has a different character from the sum of its parts.” For those parts, in a way familiar in contemporary discourse but, the OED suggests, quite new at the time, Erikson used all of the following adjectives to modify identity: cultural, ethnic, racial, religious, sexual, tribal and also emerging. Yet for all these kinds of difference , the core meaning of identity for Erikson remained in the tradition of Locke and Hume, “the ability to experience one's self as something that has continuity and sameness.”
In developing his signature-term “identity crisis,” attested in OED, Erikson boldly claimed his own terrain: “The study of identity... becomes as strategic in our time as the study of sexuality was in Freud’s” (282).
Finally, in Wikipedia an additional key sense of the term in current usage is reported, as yet unattested in OED:
Identity politics is political action to advance the interests of members of a group whose members perceive themselves to be oppressed by virtue of a shared and marginalized identity (such as race, ethnicity, religion, gender, and sexual orientation). The term has been used principally in United States politics since the 1970s. Identity politics is a phenomenon that arose first at the radical margins of liberal democratic societies in which human rights are recognized, and the term is not usually used to refer to dissident movements within single-party or authoritarian states.