other
The mirror stage sets up an image of the ego as an Ideal-I for the subject. This Ideal-I becomes an "other" within the subject's experience of his or her "I", a component of a "self" that is internally divided.

Other
The Other represents "other people," other subjects whom the individual encounters in social life, but for Lacan it also stands for language and the conventions of social life organized under the category of the law. Because language and the codes of human societies pre-exist any individual human being, these systems are "other" to the individual subject. The fact that subjects, themselves internally alienated, must employ the Other of language and the law to interact with other subjects is crucial to Lacan's theory of the psyche as well as to its practical application in therapy.