For Lacan the imaginary is one fo the three orders that structure human existence, the others being the symbolic and the real.
The child is literally captivated by a specular other thanks to an identification which is also an alienation.
The imaginary is thus the realm in which self and other merge, and in which identity is grounded in a mere semblance of unity.
The imaginary is not a stage which is overcome or transcended, and is a cosntant presence in the lives of human subjects.
It arises with the mirror stage but extends far into the adult individual's experience of others and of the external world.