Invisible Enemy is a mode in the 2002 remake of Resident Evil. It is unlocked after completing Real Survival or completing the game on Normal or Hard difficulty with both Chris Redfield and Jill Valentine. In it, all enemies are, as the name suggests, invisible.
This mode made a comeback in Revelations 2 in a similar manner.
The concept was the brainchild of Director Shinji Mikami, who thought up the mode as a challenging way to force players into changing their strategy, as well as establishing a more Horror-esque environment where the gift of vision does not help.