The term עכו''ם appears frequently in halachic literature, and it is an acronym of עובדי כוכבים ומזלות, which literally means "worshippers of stars and omens." However, it seems that in most contexts it should mean something closer to "non-Jew" or "gentile."
For instance, the Kitzur Shulchan Aruch uses the term when laying out the halachos of food made by a non-Jew.
The term also appears in censored versions of Rambam's Mishneh Torah, but obviously it is acting as a replacement for נוצרים.
This question is not asking for the translation of a Hebrew term, but how it should be interpreted when reading halachic literature.