Enophthalmos refers to the posterior displacement of the globe in the orbit. It implies that the globe itself is normal and is caused by either one or a combination of 1:
structural alterations in the bony orbit
retraction
Specific causes include 2:
orbital blowout fracture: most common
orbital metastasis: breast cancer orbital metastasis most commonly produces a desmoplastic response leading to retraction, considered one of the earliest signs of metastatic breast cancer
maxillary sinus mucocele
orbital varix: paradoxically causes posterior displacement of the globe when the varix empties
congenital
It may or may not be associated with hypoglobus.
Exophthalmos (or proptosis) is the opposite, displacement of the globe anteriorly.