Lacan uses the word "ego" in his work focusing on the "mirror stage" (2). According to the OED, "ego" can mean:
"1. That which is symbolized by the pronoun I; the conscious thinking subject, as opposed to the non-ego or object. Also humorously, for ‘self’."
"2. In speech: I, the speaker. Hence
The OED lists a few other definitions that are also related to Lacan's intended meaning, but I believe the most effective to use in this instance in which he uses the word "ego" is "the sense of one's identity or self gained from the results of self-perception and external perceptions of oneself."
This Lacan does use "ego" to refer to self, as in the first two definitions, but this last "sense of one's identity..." works with his statement of "the agency of ego, before its social determination...by which he [the self] must resolve as I his discordance with his own reality" (2).
The OED speaks of using "external perceptions" to determine one's identity. At this point in Lacan's argument, he states that the self is exposed to his own image without the realization that "the other" exists. Therefore, he must come to terms with his "ego" or identity before social influence can teach him how to view himself. He must create his own reality.
Lacan's description of the "mirror stage" and mimesis asks how one can truly determine his identity. Is mimesis or one's mirror image an "other" or entirely oneself? Is true reality the seeing of oneself without any social comparisons or with the full exposure to society and others?
Oxford English Dictionary