Hi
well, it would be helpful if you also posted the context in which this term was found. Since you mention narrowband, I assume you are working with wideband signals. These two types of bands refer to frequency bands, and narrowband interference refers to the interference caused from a narrowband system to a wideband system which may be defined in a band which includes the operation frequency of the narrowband system. The opposite is also possible interference to a narrowband system under the same assumptions.
cheers
Sal