twilight zone

noun

1. the lowest level of the ocean to which light can reach

   noun.object

   Source: WordNet 3.0

2. the ambiguous region between two categories or states or conditions (usually containing some features of both)

   noun.attribute

   Examples

   But there is still a twilight zone, the tantalizing occurrences that are probably noise but might possibly be a signal.

   In the twilight zone between humor and vulgarity.

   Source: WordNet 3.0